If you are needing to pass certain parameters included in the URL of your Optin Page to the next page (e.g. Thank You or Sales page) once a user opts in, then simply follow these steps.

• Go to Campaigns in the left-sidebar menu and then select the relevant campaign.
  Open the Optin Page you would like to edit.
  
• Once opened, click Form Settings in the left-sidebar menu.
• Click on the More Options, simply switch the toggle on the Pass URL params data
• Once you are done, click on the SAVE button at the bottom of the settings.

That’s it, now when a user opted-in the URL parameters will be passed to the next page.