Ah, got it. Thanks for the extensive explanation! I don’t have the Airtable field with the webhook URL and the parameters as these are user-specific. As a result I don’t have an ‘External URL’ to redirect to.
With this in mind, I have used the approach as detailed in Redirect via Webhook After Form Submission - #5 by Nickshas instead. The custom script awaits the webhook response, and redirects the user to the new location instead.