Javascript - Get lookup guid of a field on on the user card for the logged in user.

On the user card where you find the users (firstname, lastname, mobilephone, email etc.) I have a custom field that has a lookup to another entity.

I want to get the GUID of this lookup value for the user that is logged in.

I can't find the lookup field when looking at what in "userSettings" with below code.

var userSettings = Xrm.Utility.getGlobalContext().userSettings;
 
Xrm.WebApi.retrieveRecord("systemuser", userSettings.userId, "?$select=fullname").then(    function (result) {

How do I get the guid of the value that is sett on the logged in users usercard.