Cannot register as SIP user agent with authuser@domain


I use asterisk version 1.8.17. I would like to register user agent to my SIP Provider.
Unfortunatly my provider requires authuser with domain (“authuser@domain”), but asterisk only supports next format (from sip.conf)
register => [peer?][transport://]user[@domain][:secret[]]@host[:port][/extension][~expiry]

Is there any way I can register to SIP provider with authuser@domain?