Waking up this oldish thread because I can’t get it to work either. I used the reference that gaaned92 pointed out, and it seems really simple, but it doesn’t work. I also tried the double quotes that DenB recommended and still no luck. Is anyone else using Affinity Photo as their external editor? I’m using version 5.4 of RT on Windows 8.1. (I have Affinity set up as “external editor” in Faststone Image Viewer and it works fine.)
Edit: Figured it out so I’ll post it here for others. When you’re in Preferences, the External Editor section, there is a little “button” on the left by each line (GIMP, PS, or custom). You have to click the button in order to set that editor, but you DON’T have to click the button in order to TYPE in the file path for the program! In my case, the button for GIMP was selected, meanwhile I typed in the pathname for Affinity and hit OK and it did nothing. Once I clicked the button next to “custom” and hit OK, it worked.