Input of "Format" to the ToString function

I am turning a string of unformatted data into a formatted string using the ToString function. I need to know what/how the source is used to “feed” the function with the required string format.
Please see the attachment.

If you’re not worried about the format, use the other “ToString” function. If you need to specify it, you can set it from a constant value transformation as shown in the image below. Keep in mind that you can also use an expression transformation that will let you write everything inline if that is your preference.