I am afraid there is some confusion about your needs, please provide more details to let us provide the appropriate workaround:
Where do you want to choose the domain, when creating the item or when you are in the Reassign approver?
Currently, when we configure Reassign to in Approver center, we can directly search for related members of the group, which seems to be different from the method you describe.
In addition, if you choose domain when creating the item, how to get the related approver email through the specified domain, is it lookup column?
A more common practice is to create a column of person type in the list, and then specify approver when creating the item. Because it is a field of type person, it can only search and specify members in the organization.
Community Support Team _ Barry If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.