var salesrep = nlapiLookupField('employee', -5, 'custentity_mos_default_salesrep')

var salesrep = search.lookupFields({
        type: search.Type.EMPLOYEE,
        id: -5,
        columns: 'custentity_mos_default_salesrep'
    }).custentity_mos_default_salesrep[0].value;