Which hook is triggered when user is logged in and submits the form of free trial membership

Hello all,

I am looking for the s2member hook when a s2member pro stripe form is submitted. I have a stripe pro form and I have set it for logged in user to have 1 week free trial membership.
What hook gets fired when this form is submitted? Could anyone please let me know. I want to perform some action when this form is submitted.

Thanks in advance.
Prashant