If you want to send paypal customers to a customized thank you page after checking out with Paypal, you can do so using one of the two methods below:
When adding a "Membership Order Button" element to your sales page, you can simply put the full ulrl starting with the http:// into the "success url" field. This will only work for new customers that are paying something, and currently is not meant to be used for trial periods that do not charge anything (same also true for Method 2, unless using Pro Forms).
This method works when using either a Paypal Pro form, or a Paypal Button Shortcode.
You would simply need to add success="http://yoursite.com/path-to/thank-you-page/" to your shortcode, replacing the url with your own.
success="" Success Return URL ( optional ). optimizeMember handles this automatically for you. However, if you would prefer to take control over the landing page after checkout ( i.e. your own custom Thank-You Page ), you can. If supplied, this must be a full URL, starting with http://. Your Success Return URL is only applied to (new) Customers.
Important Note (for both methods above): You can not use any pages protected by OptimizeMember, or the home page of your site in the success url as those will not work.