^^ that technique won’t work with PJSIP trunks, as the B dial parameter is on the calling channel, not the called channel. You would want the b parameter, but you can’t use that either because FreePBX is already using that internally.
Adding custom SIP headers has been a moving target for years. Most of the techniques I’ve posted in the past are all broken now. The current supported way of adding channel agnostic custom SIP headers is using the method in this post: Custom SIP header on outgoing calls