How to add email body into BusinessMail object

I’m trying to add an email into IFS Cloud programmatically.

✅Using https://ifspsd2-d05.demo.ifs.cloud/int/ifsapplications/entity/v1/BusinessMailEntity.svc allows me to insert my email header with create@BusinessMailSet.

❌When I try to include the Body or HtmlBody parameter, I get a BadRequest error.

How do I add my email body to the new header entry?