Fetch some data when user enters some other data

I have a popup dialog like this:

• Red and green sections are editable but the blue one is read-only
• Blue section content is fetched from the IP and port previously entered by user
• Green section is entered by user according to the content of blue section

Now I wonder:

• Is there a better UI/UX design for what I intend to do?
• When to fetch Available IDs according to IP and port? For example, when users finishes editing IP and port?
• Is it more preferable to develop a button for fetching IDs by user click?