2 Ways: Alt+G, then click (can be changed to a different key in the keybindings) Ctrl+Alt+Click (can be changed to a different key in the keybindings)
Click on the ping icon next to the minimap, then click.
The clicking can be done either in the game window, or on the minimap.
The Keybindings can be found under: Menu->Options->HotKeys->Global->Unit Management
And either: Ping Minimap Quick Ping Choose Ping Target
Also, you can click directly at the game window, no need to go to the minimap (even though the keybinding says "Ping Minimap"
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Two ways: Click the Ping icon (looks like radio waves, sorta like ((•)) ), then click anywhere in the minimap.
Press Alt+G on the keyboard, then click anywhere in the minimap.
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1) You can hold alt and click on the minimap. 2)
Hold alt + g and click on the map.
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alt + Click or ⌥ + g + click for Mac
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You can modify the keybindings in the menu - for example, I changed my ping back to ALT
+
Left-Click.
Patch 1.5 also modified the behavior so that you could ping your minimap by ping-clicking on the actual playfield you're looking at.
Thus you no longer have to mouse down to the minimap, find your current view location, and then ping
- you can simply ping-click the main playfield in front of you and the ping appears on their minimap at the correct location.
gaming.stackexchange.com/users/31909/bfishman
In patch 1.5 Blizzard changed the default keyboard mapping such that holding alt and clicking on the map or minimap no longer worked.
To revert this change, load up the hotkeys dialogue and look for Quick Ping in Global - Unit Management and simply change this back to alt.
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Nothing to add to the title of the question.
How to signal on the minimap in Starcraft 2?
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Alt+G then click, or click the Ping icon and click anywhere in the map. There are two key combinations you can use,  Ctrl+Alt+Click and Alt-G, however you can also use the Ping icon and then click.
There are two keyboard shortcuts suggested, Alt+G and Ctrl+Alt+Click, as well as the ping icon and then click.