How 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(Dimensions),
    _formcontrol);

    // Construct query on the table,
    // whose records you want to show as lookup.
    query = new Query();
    qbd = query.addDataSource(tablenum(Dimensions));


    qbd.addRange(fieldnum(Dimensions, DimensionCode)).value(enum2str(SysDimension::Niveau_I));

    // add the fields to the lookup list
    sysTableLookup.addLookupfield(fieldnum(Dimensions,DimensionCode));
    sysTableLookup.addLookupfield(fieldnum(Dimensions,Num));

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