Vitor, is there some way I can send you an invite code for Arc Browser so that you could try adding support? I tried messaging you via the forum PMs but it says you don't accept messages. I'd post the code here but it's only good for 5 invites and I'm afraid they'd get used up before you got to use it.
FWIW, I tried to figure it out and was going to submit a PR but couldn't get it working. I tried just duplicating the workflow and adding "Arc" to the "chromium_variants" line in run_bookmarklet.js but after doing so, the workflow wasn't pulling up the Add Bookmark overlay.