You can access Exchange Admin Center> Left Pane> Recipients > Shared\Mailbox > click more option to make the alias field available.
Alias is the key parameter of a shared mailbox to make it unique in the backend specially if you have multiple domain.
To explain further, here is an example. I have two domains(domain A and domain B) and want to create a shared mailbox for both respective IT teams hence
it@domainA.com and it@domainB.com. By design the address name “it” will be unique address in the entire tenancy hence you need to mask it with an alias for
the system to know the two “it” mailbox is unique from their respective domain and so that the creation will push through.
In conclusion, the IT mailbox for my Domain A will have an alias of “ita” and Domain B will have an alias of “itb”.(See screenshot for more info)
Should you have one, click here