Lookup in Form with string field

Under Form Field in lookup override Method

public void lookup()
{

    Query query = new Query();
    QueryBuildDataSource _Querybuilddatasource;
    QueryBuildRange _querybuildrange;

   SysTableLookup systablelookup = SysTableLookup::newParameters(tableNum(PurchTable), this);
    SysTableLookup.addlookupfield(fieldNum(PurchTable, PurchId), true);
    super();
    _querybuilddatasource = Query.adddatasource(tableNum(PurchTable));
    _querybuildrange = _querybuilddatasource.addrange(fieldNum(PurchTable, PurchId), true);

    SysTableLookup.parmQuery(Query);

    SysTableLookup.performformlookup();


}