CiviCRM Ajax API - Searching by Tag
you are almost there: instead of using the tag name, use the tag id (eg 5 for Volunteers)

CRM.api3('Contact', 'get', {
  "debug": 1,
  "sequential": 1,
  "return": ["tag"],
  "tag": "5"
}).done(function(result) {
  console.log(result);
});


And for performance reasons, we recommend to always set the return param with the list of fields you actually need (by default, it returns way too many that you don't need)

Otherwise, yes, it should work with the name of the tag instead of the id, but given that tags are one of the few entities that don't have an internal name that only the dev can change, I'd rather rely on the id rather than on the visible name that your users can alter