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If someone lands on a page with restricted content, how can they register for membership and immediately be redirected back to the page they were wishing to access?
This is not something directly supported by s2Member, but you could possibly set something up for this to work. s2Member does pass the URL that the User was on previously to the Membership Options Page in the URL when they are redirected. These variables are called MOP Vars, and you can find info on using MOP Vars here:
Dashboard -› s2Member® -› API / Scripting -› Membership Options Page Variables (MOP Vars)
If you’re using a custom page on success, you could possibly pass this variable via a custom attribute in your Pro Form, and have Users automatically be sent to this page when they log in the first time through some custom code.