How do I add "123" to the beginning of a string and pad it to be exactly 12 chars?

Well, you could use:

string result = "123" + text.PadLeft(9, '0');

In other words, split the task in half - one part generating the "000028431", "000000987" etc part using string.PadLeft, and the other prefixing the result with "123" using simple string concatenation.

There are no doubt more efficient approaches, but this is what I'd do unless I had a good reason to believe that efficiency was really important for this task.

var result = string.Format("123{0}", number.PadLeft(9, '0'));

You could try:

var str = String.Format("123{0:0#########}", 28431);

or

var str = String.Format("123{0:000000000}", 28431);