reating a ActivityParties for a Custom Activity inside a PCF

[JohnBrown]

I am attempting to create activity parties for a custom activity in a PCF using the webApi. Unfortunately, I continue to encounter an error around the activity parties. 

"An error occurred while validating input parameters: Microsoft.OData.ODataException: An undeclared property 'partyid_systemusers' which only has property annotations in the payload but no property value was found in the payload. In OData, only declared navigation properties and declared named streams can be represented as properties without values. "

CreateMessage(description:string, title:string, userId:string, contactId:string){
        var activityParties = []

        // From - Participation Type Mask = 1
        let Sender:ComponentFramework.WebApi.Entity = {};
        Sender["partyid_systemuser@odata.bind"] = "/systemusers(" + userId + ")";
        Sender["participationtypemask"] = 1;
        activityParties.push(Sender);
        
        // To - Participation Type Mask = 2
        let Receipent:ComponentFramework.WebApi.Entity = {};
        //Receipent["LogicalName"] = "activityparty"
        Receipent["partyid_contact@odata.bind"] = "/contacts(" + contactId + ")";
        Receipent["participationtypemask"] = 2;
        activityParties.push(Receipent);
        
        let message:ComponentFramework.WebApi.Entity = {};
        message["description"] = description;
        message["subject"] = title;
        message["activity_pointer_new_Message"] = activityParties;

        this.props.WebApi.createRecord('new_message', message).then(
            function success(message:any) {
                console.log("Message has been created");
            },
            function Error(e:any) {
                console.log(e.message);
            }
        )
    }

Any ideas on what I am doing wrong here?