Hi Moving_From_Gmail_For_Domains,
It is very sad to hear you have given up. As far as I know, currently Exchange Online only support to set up SendAs permission via PowerShell. If you require a GUI solution, I’d like recommend you submitting a feedback regarding to the issue. Your useful ideas and feedback will help us improve our products and service. Your time and efforts are highly appreciated.
If the service is requested frequently, the product team may include it in the future releases.
Thanks,
Kent Gu
