Updating Collection entry via API

Hi - very new to Cockpit as I’ve been asked by a client to do a bit of work on an existing site.

I am trying to update a collection entry via a HTTP request:

https://mydomain.com/api/collections/save/member/?token=xxxxxxxxxxxx&_id=xxxxxxxxxxx&first_name=ted

But nothing is updated.

If I use:

https://mydomain.com/api/collections/get/member/?token=xxxxxxxxxxxx&_id=xxxxxxxxxxx

I see the entry’s data in the browser as I’d expect.

I’m sure I’m missing something really obvious! Can anyone point it out please!

Thanks