You might use ' hustler ' for someone who puts something in your pocket without you asking for it and then asks to be paid for it: Hustle To push or force one's way.
To act aggressively, especially in business dealings.
To obtain something by deceitful or illicit means; practice theft or swindling.
ell.stackexchange.com/users/5763/Dan C
You can use the following words to describe someone who sells goods on the street, rather than in a shop: Hawker - A travelling salesperson who tends to shout to advertise their wares Peddler (AmE)/ Pedlar (BrE) -
A travelling salesperson Huckster - (Old fashioned)
A salesperson, generally of small items who may employ "showy" or aggressive tactics to sell their goods.
Street vendor - Someone who sells goods on the street.
Street seller - See "Street vendor".
None of these words require the selling to be aggressive, however.
I can't think of any one word that you could use to define an aggressive street seller.
ell.stackexchange.com/users/51806/SteveES
The action you describe is often more associated with a street "vendor" than a shop.
People cleaning your car windshield uninvited at a traffic light and then demanding money is another example.
However, it isn't limited to street vendors.
For example, there are restaurants, particularly in areas frequented by foreign tourists, where they will leave an unordered, apparently complimentary, dish on the table, and then charge for it.
"Forceful" is sometimes associated with this but isn't necessarily the fundamental method of manipulation.
What you describe is a form of fraud, or trying to get money through deceit or manipulation.
These people may escalate to more assertive tactics if the ploy, alone, doesn't work--righteous indignation, trying to embarrass you, aggression or other attempts at intimidation, etc.
Dan C's "hustler" is a good term.
Another is scam artist .
A "scam" is a fraudulent or deceptive act or operation - M-W "Con artist" is related, but is typically associated with something more complex, involving multiple steps to "set up" the person being taken advantage of, while a scam can involve just a brief encounter, like your example.
A similar term that is a bit dated is flimflam : deception, fraud - M-W .
A scam artist or con man used to be called a "flimflam man" or "flimflam artist".
ell.stackexchange.com/users/48032/fixer1234
Pusher - a person who too readily or forecfully promotes the use or purchase of a particular thing.
(Google definition).
Haven't seen this one in the answers yet
ell.stackexchange.com/users/52412/RobPio
A tout
The verb " to tout " can mean "to solicit, peddle, or persuade importunately"
As a noun it means "one who touts: such as [...] one who solicits patronage"
ell.stackexchange.com/users/None/
I think hustle is correct.
He is a 'fast talking street hustler' trying to 'con you into' buying his flower.
Speed is his friend because, if you take time to think about it, buying the flower will just encourage the same person to pull the same stunt next time they see you or anyone like you
and you're better off nipping it in the bud!
hus·tle
verb 1.
force (someone) to move hurriedly or unceremoniously in a specified direction :
they hustled him into the back of a horse-drawn wagon
ell.stackexchange.com/users/52399/kztd
You can also combine some of these words, as in " street vendor scam artists ", " scammy street hustlers ", "*hustler scam artists", " high-pressure street vendors ", etc.
ell.stackexchange.com/users/49800/Stew C
Planter.
Actually, that's not a word that is commonly used for a person doing that.
However, a guy put a flower into my pocket and asked to pay for it.
the verb "plant" is commonly used for such a thing.
Especially in terms like this: The police officer planted evidence
That sort of usage of the word is quite common.
(If he accuses you in front of other people, then he chose to "frame" you.
That implies an accusation of something you are innocent of.)
Some other possible words: swindler (kind of derived from part of Dan C's answer ), fraudster, criminal, crook
ell.stackexchange.com/users/17653/TOOGAM
What is the word meaning people who sell something in forceful way on the street or places?  For example, a guy put a flower into my pocket and asked to pay for it. I wonder I can say street seller. But  street seller  means people who has their stores on the street, doesn't it? He has no store or shop. He just carries many flowers and sell them in forceful way.   What is "the word" meaning people who sell something in this way?
What is the word meaning people who sell something in forceful way on the street or places?
ell.stackexchange.com
<word-meaning>
ell.stackexchange.com/questions/124792
ell.stackexchange.com/users/48183/Ting Choe
Many words can be used in this context, including  hawker, peddler, huckster, salesperson, street vendor, street seller. However, these words to not imply any aggression, so you could say street vendor scam artists, street hustlers, or high-pressure street vendors, for example. Someone who sells goods on the streets is known as a street vendor.
The following words can be used to describe someone who sells things on the street. In order to sell their products, a salesperson may use "showy" or aggressive tactics. Someone who sells goods on the streets is known as a street vendor. "Street vendor" is a synonym for "street seller." You can also use phrases like "street vendor scam artists," "scammy street hustlers," "*hustler scam artists," "high-pressure street vendors," and so on to combine some of these words.