Hello,
I'm WHMCS admin user/developer already since v2.2 (somewhere in year 2006), did many template customizations, hacks & hooks, server provisioning modules and payment gateways (My last modules were Bitcoin & Ethereum payment gateways as well as VPN provisioning for WHMCS 7.10.0).
Developing a module for creating an account on a forum would be a piece-of-cake for me.
Lets chat & brainstorm your future WHMCS module!
cheers,
Junell