optin date in fluent crm

Hi, I'm using fluent forms pro and fluent cam pro. I'm doing lead generation with fluent forms, and every time a user become a lead is tagged with a tag. Now I wanna have the optin date. How can I do that, considering that sometimes even old leads of old funnels opt in in this new one? I created a hidden field and put data inside. How can I pass this value to a custom field inside the fluent crm?