fill 2nd combobox by choice 1st combobox

This is really easy, you just add 2 StringEdit boxes to your form, and you have to
select the correct EDT for each control. For the employee box it should be ProjResponsible
and for the Project box it should be ProjId. On the Employee properties you need to enable
the 'Auto declaration' option.

And you need to overide the lookup method for the project box.
It should be something like this

public void lookup()
{
    TableId tableId =       tableNum(ProjTable);
    Query                   query = new Query();
    QueryBuildDataSource    sbds;
    SysTableLookup          sysTableLookup = sysTableLookup::newParameters(tableId, this);
    ;
    sbds = query.addDataSource(tableId);
    sbds.addRange(fieldNum(ProjTable, Responsible)).value(Empl.text()); // given that your employee box is named 'Empl' and the field you filter on is 'Responsible'

    sysTableLookup.parmQuery(query);

    sysTableLookup.addLookupfield(fieldNum(ProjTable, ProjId));
    sysTableLookup.addLookupfield(fieldNum(ProjTable, Name));

    sysTableLookup.performFormLookup();
}