Create Account Record with Lookup Field Value using Web API

I am creating an Opportunity using the new Web API REST endpoint and I have the following code to do so

var entity = {};
entity.name = "ABCD";
entity.customerid = {
            Id: "CC7CC89D-3A4D-4BA1-9255-5DC2A92856DB",
            LogicalName: "account"
        };

var req = new XMLHttpRequest();
req.open("POST", Xrm.Page.context.getClientUrl() + "/api/data/v8.0/opportunities", true);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.onreadystatechange = function () {
    if (this.readyState === 4) {
        req.onreadystatechange = null;
        if (this.status === 204) {
            var uri = this.getResponseHeader("OData-EntityId");
            var regExp = /\(([^)]+)\)/;
            var matches = regExp.exec(uri);
            var newEntityId = matches[1];
        }
        else {
            alert(this.statusText);
        }
    }
};
req.send(JSON.stringify(entity));

My problem is I can't include a lookup field in the create, every time I do so, I am getting an error.

What is the correct format to include a lookup field in create?

Thanks a million!!