How to call js + Ajax from onclick in .apsx?

Hi,
I'm working ona webforms app that was written 5 years ago. for 1 page, I need to add a function where when a user clicks a radio button, I need to go back to the server side, access db, do some calculation, then display a message to this user via a pop-up; BEFORE this user can procedd and go to next page.
I'm thinking that I should add a "onclick=..." to the radio button, which will run a javascript, which will make an Ajax call then I can access the db, do the manipulation later, then return the result and display it in an "alert" window.
Currently the radio button code in .aspx looks like this:
<asp TemplateField>
    <ItemTemplate>
          <input name ="ServiceSelected" type ="radio"
                     ...