The participation type mask should be 2, which is a To Recipient. You will need to specify the recipient's user record for the activity party's partyid lookup in OData endpoint syntax, e.g.

{
  "ToRecipients": [{
    "participationtypemask": 2,
    "[email protected]": "/systemusers([my systemuser guid])",
    "addressused":"[email protected]"
  }]
}

This is the same syntax used when setting lookup columns in a Dataverse create/update action in Power Automate.

References

Participation Type Mask values: https://learn.microsoft.com/en-us/power-apps/developer/data-platform/reference/entities/activityparty#participationtypemask-choicesoptions

Setting lookup values in Web API operations: https://learn.microsoft.com/en-us/power-apps/developer/data-platform/webapi/create-entity-web-api#associate-table-rows-on-create