Your question is not clear as to what you are trying to do.
"must be banned! Add specific users to a custom field" does not make any sense. Even if we break it down, "must be banned" has no context, although "add users to a custom field" does it we assume you have multi-user-picker and want to add people to it (that's a line of code to set it, but you have not told us anything about who to add).
Sorry for my english) I have a field called (witnesses) in which there are responsible users who are responsible for permissions to provide access to systems like AD pressure or Jira I need to do so as soon as the responsible user in the request clicks on the (approve) button is removed from that list (witnesses)
I'm afraid that does not help me much, could you break it down so it is not all one sentence?
You have a field called witnesses which is a multi-user picker. After that point, I do not understand what you want to do with it. (Forget what the field is for, just a simple description of what it should do)
