add another constant in Membership Pings Editor

I'm setting up the remote ping with Mailchimp API for a website using Membership plugin and almost have it working.

I want to use merge_vars[FNAME]=
and link it to the actual users "Name" that is entered, not the %username%

spoke to @Alexander Rohmann and he suggested maybe:

"The developer included a hook to easily allow for adding custom fields, so if you open a forum thread, I can see if he can get in touch with you there about it"

Thanks for your help