Hey there! You can either add an action above or below the If action, and then simply click and drag it into the appropriate spot, OR you can click into one of the areas that says "No Action" (in your screenshot the upper one is highlighted) and then use ⌃⌘A to insert an action by name directly in there.
There's probably other methods but that's what I use most.
That was the first thing I tried (It’s a simple acton, just do down arrow and then ⌘-9) but all I get is a red box outlining the No Action block. Trying to manually drag the action “type a keystroke” also does not seem to work.