Using Account / Subaccounts wihtin request

We can set up new users within a request (rep phone call) having its name, email, account, phone number, etc.

But if we have sub-accounts then whe have the problem, we can then change contact information and allocate them to their corresponding sub account but cannot chage the request to the propoer subaccount (bussiness unit) he/she is at.

Under request form customization we do not see any place where sub-accounts can be selected. We can olny see and select accounts.

Can a new form be designed because for new customers/accounts is easier that new users are registered as they make new requests under their account/subaccount.

Having the request we cannot modify to select the subaccount too.