Error is raised when I try to redirect webform submission to external webpage with query string
I think there's an easier way to achieve what you want.

In you webform go to "Settings > Form" then you'll see near the bottom "Custom settings" with a dropdown for "method". There you can change it to "GET (Custom)" then enter the url for your third party site.

If you needed the get variable names to be something specific you might need to remake your fields with the right key to match your get variable names you need.

But that should do the trick and it will take care or encoding any special characters for you