1- To redirect your users to external url, you have first to install and activate the thank you screen addon:
2- After activating the “Custom thank you screen” add-on, go to your form then click the plus icon and choose the thank you screen block:
3- Click on the settings icon for the thank you screen then check the “Auto redirect” toggle and type the redirection url in the text box:
You can have more than one thank you screen and each one has a different redirect url. You can use the jump logic to jump to any thank you screen based on logic.