BAM and Widgets

I am trying to write a BAM that references the Purchase Order Entry
screen. When the "Approve" box is toggled, and the Buyer limit is
exceded, a pop-up box appears to let the Buyer know he needs to get
approval from a higher level. I want to send an e-mail to the
POApv.MsgTo person if YES is selected in this box. Where can I find a
reference on how to code this using what I think is a reference to the