Is there a way to create a key command to put a border around a text?
Not as far as I know, but you can design a paragraph style with a border, set it as a default (=star) and assign a key command to that, works great!
B.
Can you please explain (to this relatively new user) what you mean by "set it as a default?
Set as default is probably the wrong way to put it, but once you have created the style the way you want it, click the little star symbol on the bottom!
This way, the style becomes available in all future projects, and you can also assign a key command to it in preferences.
B.
Meaning I can assign a key command to that specific style? Or to the styles in general?
No, to that specific style!
Hi @dfelsenfeld, the custom Paragraph Style with integrated activated border is a nice and consistent method. But there is another general method (that you can use also for different scripts): following the method that you can find in this post, I made a video explaining how to apply a custom shortcut to a recorded script that will add a border to any text-item. (There are other methods, using MacOS integrated menu shortcuts, or keyboard maestro, etc…, but this method works without needing third party tools 
):