Hello Jun Wei,
Thank you for posting in Q&A forum.
I was prompted unable to do that due to no permission for editing, how do I get the permission from Domain B to add the Administrator role inside Domain A environment?
A: You can try to let administrator in Domain B give you such permission (give you to modify group membership in domain B), then you can add demo01 to Administrators group Domain B.
Or you can let administrator in Domain B add demo01 to Administrators group in domain B.
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.