Include in custom form for Vantage Directory theme

Hello,
I’d like to know how I can integrate BAW Easy Invitation Codes plugin with my theme, I want it to function as an invitation only directory.

https://www.appthemes.com/themes/vantage/

http://demos.appthemes.com/vantage/

http://demos.appthemes.com/vantage/create-listing/

the invitation code will be at the bottom of the create a listing page.

Please advise.

Thanks for all your help.

https://wordpress.org/plugins/baw-invitation-codes/