Override lookup method of Form Control

public void lookup()

{

SysTableLookup        sysTableLookup;

Query                 query=new Query();

QueryBuildDataSource  qbds;

;

sysTableLookup=SysTableLookup::newParameters(tablenum(MyTable),this);

sysTableLookup.addLookupfield(fieldnum(MyTable,Field1));

sysTableLookup.addLookupfield(fieldnum(MyTable,Field2));

qbds = query.addDataSource(tablenum(MyTable));

qbds.addRange(fieldnum(MyTable, UserId)).value(curExt());

sysTableLookup.parmQuery(query);

sysTableLookup.performFormLookup();

}