Access Parent Window through VBA in Microsoft Dynamics GP

Hi All

I am customizing Assembly Serial/Lot Entry window which appears when we click Serial/Lot button on Assembly Entry window to automatically generate serial numbers through VBA customization. To generate serial numbers we need document number of assembly entry and for that purpose I need to know how can I access parent window to know relevant document number through VBA. 

Will be grateful if anyone of you could let me know how to do this.