Associate a message with a campaign, which is the association a split (A/B) campaign uses to send more than one message.

campaignid is required; a request without it is a 400.

Only campaignid, messageid and percentage may be written. Every other field of a campaign message is a counter maintained by the sending code, and is ignored when it is submitted.

The request needs the campaigns feature to be enabled for the account, and the caller has to have access to the campaign, otherwise the request is a 403. A body without a campaignMessage object is a 400.