To setup a "thank you" page for your opt-in form, from the campaign builder, click on the optin form widget and go to the settings tab in the right side panel.

If you use the "HTML Form Code" option, then the code you're using will already typically already include a thank you redirect url. In this case, Wishloop will redirect the visitor to the "Thank You" URL already provided in your form code.
If you use the "API" option, then you will be able to specify your thank you page URL in the "THANK YOU PAGE URL" field within Wishloop. Once your visitor submits the form, they'll be redirected to the URL you enter here.

Further Reading:

Showing a 'Step' (Thankyou page) after an opt-in form submit

How to setup a "thank you" page for your opt-in form?