If doing GUI scripting, it is extremely worthwhile to invest in UIBrowser:
It is US $55 and has a trial version. It will show you live if that dialog supports UI scripting, and if so it will even generate the syntax you need to reference the “Remove Now” button.
It’s well worth the time to try it out. Otherwise, we are guessing in the dark because of the way UI elements can vary.
The other easier route—if you only need to make this script work on your own system—would be a macro program which supports AppleScript. I haven’t used one in a long time, but I believe Keyboard Maestro is the preferred one now.
FWIW, I will have a short section on UI scripting, including using UI browser, at this upcoming training event: https://www.cmddconf.com
Nope—Safari scripting will not be covered. Script Debugger? Oh, yeah…I’ll do my best.