I have tried creating a new check or modifying an existing check to add a tag. However when i open the check again, there is no tag.
I also tried to use the Pingdom API to modify a check, and that seemed to indicate i need to preregister the tag, but cannot find anything about this in the documentation:
% curl -X PUT 'https://api.pingdom.com/api/3.1/checks/<check_id>' -H 'Authorization:Bearer <token>' -H 'content-type: application/json' -d '{"tags": ["staging"]}'
{"error":{"statuscode":428,"statusdesc":"Precondition Required","errormessage":"Unknown user tag \"staging\". Create it first to connect it to a check."}}
(I also tried to submit this using "Submit a case", but funnily enough that is broken)