Set up iCloud email when Apple ID not iCloud address
Context: MyApple ID does not use my iCloud email (it uses a btinternet one).
Problem: I am setting up my iMac and I want to put my iCloud email into my accounts so I receive iCloud email. However, when I try to 'add account' and select 'iCloud' it simply detects my non-cloud Apple ID email (btinternet) and wants me to use that. It won't let me put in my original iCloud email - so I can't get standalone mail from this account via mail client on iMac. Please help/advise.
PS It seems I can receive iCloud mail as this comes in via the Apple ID but I cannot selected iCloud as the outgoing email address is cannot send form iCloud email.
iMac 27″ 5K