Gravity Forms + Mailchimp SMS Opt In

My website is using Gravity Forms for an advanced web form. I am also using mailchimp for SMS Automation marketing. I am trying to get the submissions opted in for SMS but Mailchimp tells me there is not way to do it unless I use their pre built forms. That wont work for me since my form has advanced fields.

How can I get people who submit my gravity form opted in to SMS Automated marketing? The SMS messages are time delayed to go out every couple days until the sequence is completed.

Mailchimp’s API does not support the SMS phone number field when creating or updating the contact (audience member), so it’s not possible to accomplish this with the add-on.