@julian Seems to do the trick. Thanks for the pointer.
For others if you are passing multiple tags through curl here is a working example.
curl --location --request PUT 'https://forum.com/api/v3/topics/14/tags' --header 'Authorization: Bearer ffee84f8-2afd-4ab5-91d6-b57afa12a2025' --data-urlencode 'tags[]=visit' --data-urlencode 'tags[]=cambodia' --data-urlencode '_uid=1'
