A typical method of deploying a keystroke injection attack tool is to disguise it as a regular USB thumb drive, and leave it lying around near the intended victim's computer. Human nature being what it is, there's a good chance they'll pick it up and plug it in, just to see what's on there, or maybe look for identifying information in order to return it to the rightful owner.
You can do the same with your Gemma M0, if the person you want to prank has a penchant for microcontrollers, or you can sneakily plug it into their computer while they're focused on the screen. Want to be sure to get away scott free? You could add a delay to your code so it won't run for a little while after being plugged in -- make a clean getaway and then wait for the antics to begin!
Here are a couple of upgrades you can make to your Foul Fowl.
For one, you can take a short length of heat shrink tubing and encase the Gemma M0 and USB cable for a nice, stealthy package!
There, that's a bit less noticeable!
If you want a really subtle package, you can swap out the short USB cable for a Tiny OTG adapter!
If you want to try a simpler project, that's just as fun and uses MakeCode, check out the Phantom Mouse Jiggler! I'll make your victim question the sentience of their own mouse cursor!!