The Webhooks Add-On will always send data after the form is submitted, so you won’t be able to redirect the user using anything built in to Gravity Forms. The confirmation will be shown before the Webhooks Add-On processes the submission.
If you have the RedirectURL in the webhook response, you could, in theory, send an email to that visitor, with a link to the Buckaroo payment page, You can use the gform_webhooks_post_request hook as the correct time to do something with that RedirectURL:
Thank you for your reply. To make it easy for the donation form submitter I would like to instantly redirect them to the payment page, so i think i have to look at another solution for this to work. I did also contact GF support and they suggested to use the gform_confirmation filter to send entry data to third-party.
Only I also need to add a bearer token to the authorization header so I used the gform_webhooks_request_headers filter for this. If I don’t use the webhooks addon I am a little bit lost on how to add the authorization header to the form data that is send to the third party API.