How is _tags field filled for REST Services?

I have a form for which we push data via the REST Services to our own endpoint using JSON. I was hoping to use the “tags” feature to push some form-creator-specified metadata with each submission. I have added a tag via the frontend, using the “tags” icon in the Project view. However, the “_tags” list is not filled in the requests I receive from the corresponding form submissions.

Am I misinterpreting how the _tags field is used? If so, is there another way to push static, but form-creator-configurable, data with each form submission?