Use Greasemonkey to change URL links within a given domain

I'm new to Greasemonkey (Tampermonkey actually), and I'd like to write a very short script that:

• Is valid within a given domain (e.g. "mydomain.com")

• Parses all button-related URL links within the active tab

• Replaces them as follows :

  Original URL link: [string_1]/[useful_part]?[string_2]

  To be replaced with: [replacement_1]/[useful_part]

  So everything after the "?" can be discarded, including the "?" itself.

More specifically, URL links are as follows:

http://127.0.0.1:6878/webui/player/[useful_part]?autoplay=true

So string 1 = "https://ift.tt/JuzOUHN" and string 2 = "autoplay=true"

I've seen a similar question here: Rewrite parts of links using Greasemonkey and FireFox

But I'm not good enough at RegEx, so I couldn't adapt the script to my own needs.

I've also looked for Firefox extensions, but the available extensions don't seem to allow the level of text replacement that I'm seeking.