X++ code to create a customized lookup on form

Override the lookup method on Formdatasource field(on which you want to show lookup) , and copy the following code to your method.
Comment the super() method in the lookup.

public void lookup(FormControl _formControl, str _filterStr)
{
SysTableLookup sysTableLookup; // systemclass to create //customlookup
Query query;
QueryBuildDataSource qbd;

;
sysTableLookup = SysTableLookup::newParameters(
tablenum(InventTable),
_formcontrol);

// Construct query on the table,
// whose records you want to show as lookup.
query = new Query();
qbd = query.addDataSource(tablenum(InventTable));
qbd.addRange(fieldnum(InventTable,ItemType)).value(SysQuery::value(enum2str
(ItemType::Item)));

// add the fields to the lookup list
sysTableLookup.addLookupfield(fieldnum(InventTable,ItemId));
sysTableLookup.addLookupfield(fieldnum(InventTable,ItemName));

// pass the query as parameter
// system will show the records in the lookup
// as per your query
sysTableLookup.parmQuery(query);
sysTableLookup.performFormLookup();

}