How to make sql connection dialogue box on application start

Need more details. What kind of an app is it? c#, vb.net. asp.net, wpf etc?

Just having a form popup with a dropdown list full of various connection strings is what you are looking for I guess.

Okay. Assuming that you have a set of databases for which you have all the information already. Then you can make an XML file with that data. Like this...

<?xml version="1.0"?>
<DataBase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <Node>
        <Server value="gdms-dolphin" />
        <DB value="BioStar" />
        <User value="sa" />
        <Location value="Los Angeles" />
        <SMTPServer value="Exchange" />
        <EmailID value="johnsmith@gmail.com" />
        <Password value="password" />
    </Node>
    <Node>
        <Server value="gdms-orca" />
        <DB value="BioMoon" />
        <User value="sa" />
        <Location value="Newyork" />
        <SMTPServer value="Exchange" />
        <EmailID value="johndoe@gmail.com" />
        <Password value="password" />
    </Node>
</DataBase>

Then just read the XML and populate the form. To do it, just google how to populate controls from XML file. Should be simple.

Also I am guessing your server field should be a dropdown and selecting a server in it should accordingly populate the rest of the fields. This can be done on the selection change event of the dropdown list; again very easy in winform.

Good luck.

[Please encourage participation by up-voting solutions or answers that work for you]