Passing Information Back To ASP Page From Another Page

I have an application which allow users to search our database. Quite a lot
of search fields pull data from other tables. I don't want to populate the
dropdowns (total 14 of them) from the database everytime as it slow the page
load.

Instead I would like to have a link next to the text field which a
user and click and the choices can be displayed and whatever the user click
in the pop up I want to transfer it to the text box and close the popup.

For example there is a Textbox called country. The user can either type in
the country name or click on the link which will pull all the countries from
table in the database and which ever country user selects, the textbox is
popluated with the country name and the pop up closes automatically. Code: