I was able to populate the field by using this script:
// *** Function to set a party list
function setToPartyList(fieldName, id, name, entityType) {
var party = Xrm.Page.getAttribute(fieldName);
// Create new array
var partlist = new Array();
partlist[0] = new Object();
partlist[0].id = id;
partlist[0].name = name;
partlist[0].entityType = entityType;
// Set array value
party.setValue(partlist);
};
