Add feature tags API not working

Hi, has anyone managed to use the add feature tags API endpoint? (https://support.billsby.com/reference/post-subscription-tags)

The documentation page doesn't work as it enters subscriptionUniqueIds as a string rather than (I assume) an array of strings, but submitting an array of strings doesn't appear to work either. I get an empty 200 successful response and it looks like nothing happens. Has anyone else encountered this issue?

Thanks