capture variable from the popup in Desktop application

You're can save the text into a variable and then start extracting the info

If the Customer number is always in the same place then remove the "Client " part of the string.

Then if its always the same length remove everyting after the length of the customer number.
if the length is not always the same look for the position of the first " " (space) in the remaning string and remove everyting after that including the space itself

The customerName might be more tricky since it might or might not contain a space.

I would suggest finding the position of the " has been...." and calculate from there what part of the string you need.

To manipulate a string you can use the aqString methods:

https://support.smartbear.com/testcomplete/docs/reference/program-objects/aqstring/methods.html 

I had a similar issue where I would need to extract an ordernumber from a msgbox string,

Since most of the string didnt change only the number I just went step by step removing the unneeded parts of the string until only the ordernumber was left.

Using Regular Expression, you can do something like this

function test1()
{
    var messageBox = "Client 121157871 Atul Patil Sancheti has been successfully added.";

    // RegEx
    var regex = /Client\s+(\d+)\s+(.+?)\s+has been successfully added\./;
    var match = regex.exec(messageBox);

    if (match != null) {
        var clientId = match[1];    // 121157871
        var name = match[2];        // Atul Patil Sancheti

        Log.Message("Client ID: " + clientId);
        Log.Message("Name: " + name);
    }
}