1) Go to Menu->Custom redirect
2) Press on the + on the right
3) Insert the thank you page url in the URL field.
* Choose Reu = Yes
* Process URL = Yes
Press Create.
4) Check the table , you will find the new row at the top, next to it there is an rd number, this is the number you need to pass on the URL of your landing page / in the API Request's body in order for the redirection to work.
rd must be in lower case.
for example:
Add to Landing page url
&rd=94
Add to API request:
"rd":94