events manager pro

I am using events manager pro. I have an admin facility for pro booking / custom forms and want to add an opt in / opt out mailchimp subscription checkbox to this form
Do I just need to add a field name ( if so what is the correct field name ?)
Or is this a bespoke coding job, if there is a quick fix, any guidance much appreciated.
thanks
e

http://ww.wp.xz.cn/plugins/autochimp/