Create Email Activity on Opportunity via REST API email endpoint

Hi ​@SethB ,

Could you please try the steps below and check if it helps?

1. Make a GET call to retrieve the Opportunity's NoteID.

              GET /entity/Default/24.200.001/Opportunity/{OpportunityID}

2. Use that NoteID as the RelatedEntityNoteID in your Activity request.
3. Set the RelatedEntityType to:

            "RelatedEntityType": {
             "value": "PX.Objects.CR.CROpportunity"
            }

4. Ensure RelatedEntityDescription matches the Opportunity (optional but recommended).
   If you're receiving a 406 Not Acceptable, it's likely not because you're linking to an Opportunity. Since the same pattern works for Sales Orders, I'd verify:

• The GET returns a valid Opportunity NoteID.
•  RelatedEntityType is PX.Objects.CR.CROpportunity.
•  You're using the Activity endpoint (or Email Activity endpoint if applicable) with the correct entity version.
•  Required fields (Type, Status, Summary/Subject, etc.) are valid for the Activity type being created.

Sample:
url: /entity/Default/24.200.001/Activity
Method: Put
Request:
{
  "Subject": {
    "value": "Draft Email for Opportunity"
  },
  "Body": {
    "value": "This is the body of the draft email."
  },
  "MailStatus": {
    "value": "Draft"
  },
  "ClassID": {
    "value": "Email"
  },
  "IsIncoming": {
    "value": false
  },
  "RelatedEntityDescription": {
    "value": "OPPR000123"
  },
  "RelatedEntityNoteID": {
    "value": "02686d2e-7f68-f111-bb12-0e2c1f509de5"
  },
  "RelatedEntityType": {
    "value": "PX.Objects.CR.CROpportunity"
  },
  "To": {
    "value": "recipient@somecompany.com"
  },
  "From": {
    "value": "sender@mycompany.com"
  }
}