For this, we need to create a custom end of survey message with blank text and in the script tag of the end of survey message use below JS code to redirect:

window.location.href = "REDIRECT URL GOES HERE";

Thank you!

@Shashi  isn't there a way to go directly to the external page without the need to load the default page? Because many of our partners don't like that preload page.

We can change the default page with a custom message. Keep the custom message blank and just add the above script in the script tag.

Hi!

@Shashi I have a question about the code 

window.location.href = "REDIRECT URL GOES HERE";

if I have mandatory questions in the form, with this code if I press send without answering all the survey questions, the validation of mandatory questions is passed. 
Is there any condition where it says that, if the error is active, do not go to the thank you page until all the required questions are answered?

Thanks!