You don’t usually do this manually, but use Region > Edit > Rhythm Ferrret to analyze and find onsets or transients.
What edit-point are you using? Is it over the selected region?
Select one (or more) regions, then move the Edit Cursor (usually the playhead) on top, and add a transient marker. A gray line will appear at the edit-point in the selected region(s):
The main effect is that it will act as additional stop when you Transport > Playhead > Move to next/previous transient (ctrl + arrow-left/right).
It will also be used as additional split-point when you use the Rhythm ferret.
You can select and drag a transient maker with the mouse to move it, and use shift + click to delete it.