mdr2273, you can pass the information on the querystring for your popup window to read and use something like:
this.window.creator.document.frmName.fldName.value=yourvalue
to write it back to the original page.
My prefered method would be to not open up a popup but to have a DIV on the same page show up absolutely positioned so it is on top of everything else. Use AJAX to test the values and present any options then hide the DIV and write the values back to whatever fields your page needs.
It is a lot cleaner than going to a new window which may be blocked by popup blockers or end up losing focus depending on what the client is doing and going into the background.
I do the very thing you are talking about. I have a form field for a persons name to go with an onclick event. When the event fires it pops up a box asking for the ID number or the persons name. The name can be first last or last, first or even partial names. We resolve the name/id against our exchange server and if based on a name there are multiple possible matches it lists them all out and the client clicks the one they want. Then the box closes, the name is written into the field and the ID#, first name, last name and email addresses are all written to hidden form fields for later use.
At my age I still learn something new every day, but I forget two others.