I have a drop down list on the main form, I want to click the button next to that which would open a new form and have completed a drop down list (same source) with what was just selected.
when you say "drop down list" do you mean a combo box?
if that's the case then,
When you say "with what was just selected", do you mean a single item in this combo box, or do you mean the row source of the combo box?