Create a custom lookup with multiple data sources on form using X++

Open the form on which you want to create a custom lookup. Go the data source - fields, select field and override the lookup method. Following code will help you in creating custom lookup using SysTableLookup with multiple data sources. 

public void lookup(FormControl _formControl, str _filterStr)
{
     SysTableLookup            sysTableLookup;
     Query                          query;
     QueryBuildDataSource  qbds1, qbds2;
     ;

     sysTableLookup = SysTableLookup::newParameters(tablenum(table1),_formcontrol);

     query = new Query();
     qbds1 = query.addDataSource(tablenum(table1));
     qbds2 = qbds1.addDataSource(tablenum(table2));
     qbds2.joinMode(JoinMode::InnerJoin);
     qbds2.addlink(fieldNum(table2, field1), fieldNum(table1, field1));
     // if relation already defined in AOT between table1 & table2 then comment above statement use below one.
     //qbds2.relation(true);

    qbd1.addRange(fieldnum(table1, field1)).value("value");
    qbd2.addRange(fieldnum(table2, field1)).value("value");

    sysTableLookup.addLookupfield(fieldnum(table1, field1));
    sysTableLookup.addLookupfield(fieldnum(table1, field2));

    sysTableLookup.parmQuery(query);
    sysTableLookup.performFormLookup();
}