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You can use Send POST data in your custom Thank you page. Go to Preferences and on Advanced settings tab set Send Post Data to Yes
Now your form sends post data to your thank you page. Now you need to go to Thank you page.
A window will pop-up on the right side of your screen.
Choose REDIRECT TO PAGE text box will show where you can enter url to your page.
When you fill text box, press APPLY.
Please let me know if this works for you.