How about we explain what to do and you try it for yourself? Programming is about using your knowledge to write code that does what you want, not copying and pasting code snippets that you find on the web and getting other people to write the rest for you.
That fourth line is quite ugly and could be improved considerably. Firstly, all those CStr operators are useless. The string concatenation operator (&) is defined for types String and Integer and all those TimeSpan properties are type Integer. Get rid of all the CStr operators and it will work exactly the same. Some people might think that that would require implicit conversion of the Integers to type String but it does not. It is not the case that the Integers are implicitly converted to Strings first and then passed to the & operator. The operator itself accepts the Integer and explicitly converts it to a String. Secondly, you shouldn't be using & anyway. The String.Format method is specifically intended to make concatenating multiple values like that easier to read. You should read the MSDN documentation for that method and, if required, look for examples of its use online. Personally, I use it pretty much every time I would otherwise have to use more than one & on the same line.
As for the question, If you want to stop the Timer when the time has elapsed then you simply call Stop on the Timer at that time. Similarly, if you want to display a form then create a form and display it. If you don't know how to do that then I suggest that you follow the Tutorial link in my signature below and learn how. As for when to do it, what would tell you that the time has elapsed? Think about it. It's not a programming question because the same would be true if you were doing this with a pen and paper. If the current date/time is greater than or equal to your reference date/time then time has expired. That's a simple comparison. Also, if the time difference is less than or equal to zero then time has expired. That's also a simple comparison, using TimeSpan.Zero. You just make one of those two comparisons and, if time has expired, you stop the Timer and display your form.
Have a go at that and if it works then you're done. If it doesn't work then you can always post back here again, show us what you've done and explain your logic and we can help you fix the issue. The important thing is that you use your brain to try to solve the problem because the best way by far to learn is to do.
