Hey there Christian! Were you able to get anywhere on this?
Tomer, I am attempting the same thing. My goal is to redirect after a new sign-up only to go to a terms and agreement page (or lightbox). Either this, or add an agreement checkbox on the initial signup page using the Wix Members App. Similar to how the user can opt in to the community forum upon sign-up.
Hoping to meet an approaching deadline soon, if anyone out there is able to shed some light! 