Deep Insert Request for Account Entity using AJAX on Dynamics 365

The following request body posted to the Account entity set will create a total of four new entities in the context of creating an account.
· A contact is created because it is defined as an object property of the single-valued navigation property primarycontactid.
· An opportunity is created because it is defined as an object within an array that is set to the value of a collection-valued navigation property opportunity_customer_accounts.
· A task is created because it is defined an object within an array that is set to the value of a collection-valued navigation property Opportunity_Tasks.The following is the source I used for this:

https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/webapi/create-entity-web-api#bkmk_CreateRelated


var fnDeepInsert = () => {

    let entityName = "accounts";
    let clientURL = parent.Xrm.Page.context.getClientUrl();
    let req = new XMLHttpRequest();

    req.open("POST", encodeURI(clientURL + "/api/data/v9.1/" + entityName, true));
    req.setRequestHeader("Accept", "application/json");
    req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
    req.setRequestHeader("OData-MaxVersion", "4.0");
    req.setRequestHeader("OData-Version", "4.0");

    req.onreadystatechange = function () {
        if (this.readyState == 4  ) {
            req.onreadystatechange = null;
            if (this.status == 204) {
                alert("No Content = Four Entities created using DEEP INSERT");
            }
            else {
                let error = JSON.parse(this.response).error;
                alert(error.message);
            }
        }
    };

    req.send(JSON.stringify(
        {
            name: "Account",
            primarycontactid:
            {
                firstname: "Bender",
                lastname: "Rodriguez"
            },
            opportunity_customer_accounts:
            [
             {
                 name: "Opportunity for Bender",
                 Opportunity_Tasks:
                 [
                  { subject: "Task related to Bender" }
                 ]
             }
            ]
        }));
}