Solved! Thank you Toby & Richard for your replies. It wasn't an authorisation issue or a forward slash problem. For the benefit of people who have this issue with Zapier in the future -> take a look at the image attached to this post. In it, you will find a field called "Send As JSON". This should be set to "No" and then Zapier will simulate a form. The search will then work. I hope that helps future people. Thanks again to Toby & Richard.