I hope this will help you.
public void lookup()
{
Query query = new Query();
QueryBuildDataSource qbds;
QueryBuildDataSource qbdsJoin;
SysTableLookup sysTableLookup = sysTableLookup::newParameters(tableNum(SalesLine), this);
;
qbds = query.addDataSource(tableNum(SalesLine));
// use query join, if other tables involved.
qbds.addRange(fieldNum(SalesLine, salesId)).value(SalesId.valueStr());
qbds.addRange(fieldNum(SalesLine, Ignlicenseplate)).value(licensePlate.valueStr());
qbds.addRange(fieldNum(SalesLine, ItemId)).value(ItemId.valueStr());
sysTableLookup.parmQuery(query);
sysTableLookup.addLookupfield(fieldnum(SalesLine,InventTransId));
sysTableLookup.addLookupfield(fieldnum(SalesLine,ItemId));
sysTableLookup.addLookupfield(fieldnum(SalesLine,PurchId));
sysTableLookup.addLookupfield(fieldnum(SalesLine,SalesQty));
sysTableLookup.addLookupfield(fieldnum(SalesLine,SalesUnit));
sysTableLookup.addLookupfield(fieldnum(SalesLine,SalesPrice));
sysTableLookup.addLookupfield(fieldnum(SalesLine,LineAmount));
sysTableLookup.performFormLookup();
}