There are several email address guesser services available online. One of the easiest to use and free options is http://www.guesser.email/. The guesser will generate addresses based on this information. If you don't know the domain, start with the person's company domain (@company.com), then try other popular domains like @gmail.com or @icloud.com. You can enter multiple domains by putting a comma in between each one. This will open your email client with all of the generated addresses added to the BCC line. This will allow you to send the same email to each message without the person knowing that you sent it to every permutation of the address. Make sure that your message is friendly and clear, so that you aren't mistaken for a spammer. You are likely not supposed to have this email address, so you'll need to be polite so that you don't get ignored. The message you compose will be sent to every address generated by the website. Wait to see if you get a response from any of the messages. Most will bounce back with an "undeliverable" message, but one may get through.

TL;DR in Indonesian:
Kunjungi situs penebak alamat surel. Masukkan nama depan dan belakang orang tersebut, serta kemungkinan nama domainnya. Klik tombol "Create Email". Tulis surel pribadi. Kirimkan pesannya dan tunggu.