I was wondering about this, and now that i have actually tried it I'm getting a 403 forbidden (but that's another issue to debug i think), but it seems that this method shouldn't have a "slug" arg since we're already expected to provide the list ID. There's another method dedicated to slugs already, and the postListMemberCreateForListID method doesn't expect a slug either. Other similar methods also have variants especially dedicated to using slug or list ID.
I was wondering about this, and now that i have actually tried it I'm getting a 403 forbidden (but that's another issue to debug i think), but it seems that this method shouldn't have a "slug" arg since we're already expected to provide the list ID. There's another method dedicated to slugs already, and the postListMemberCreateForListID method doesn't expect a slug either. Other similar methods also have variants especially dedicated to using slug or list ID.