Unable to invite users to my Private Realms Instance

Unable to invite users with MS Gamertags since their Mojang usernames do not match.

During Mojang account creation years ago, we had to create Usernames WITHOUT spaces, Hence my daughter used Sarah_Hynson as a login name. 

Since the Microsoft merge, we had to create gamer tags which DO NOT allow _ characters so we used names like Sarah Hynson.  

Now, when I try to invite Sarah Hynson to my realm, it states the Gamertag does not exist.

I can still invite Sarah_Hynson and it goes through so she's able to join via Desktop PC only, but anything like an iPad or Andriod tablet does not allow the old Sarah_Hynson entry. They require a MS gamertag.

 

There is however an option to join a Realm via tablets with an invite code.

But I'm unable to figure out how to generate an invite code via my Desktop PC.