Inserting comma into a string

For example:
Var numberOne = 20
Var numberTwo = 500

myTextBox.text = numberOne*numberTwo

How would I get it to appear in the textbox as:
10,000

instead of;

And the numbers will vary in the textboxes, so I need it to be flexible for 3 digit numbers up to probably 5 or 6 digits at the most. Any ideas?? I did research, but can’t quite find what I need. Thanks.