To use Thank You page admin needs to set the page from settings.
Add Short Code
[mf_thankyou]to the page you have selected for Thank You page
After payment action done the Thank You page will something like this
You can ad more functionality to your thank you page. Here is more short code to make a awesome thank you page.
If you want to show name along with the default thank you message then pass the
fname with the field name
If you want to show the the payment status then you can use the this short code
If you want to show transaction ID then you can use this short code.
If you want to show any filed data / value from the form then follow this short code