Add a creative to a campaign. A campaign serves only once it has a creative it can serve.

Needs the campaigns:write scope, like every campaign write.

A creative is sent to DelivrDSP when it is active and its campaign has launched. One added to a draft campaign is kept until launch, which activates the campaign's draft creatives. One added to a launched campaign is sent as soon as it is active.

Image, video and audio creatives point at an uploaded file: upload it with Create creative upload and Confirm creative upload, then pass the file_id. HTML and NATIVE creatives carry their content in metadata instead, and a file_id is refused for them.

Unknown fields are refused. Refusals are JSON with an error message and a code.