Format String
To build a custom format string you can use the format specifiers provided by Microsoft. The format specifiers are listed here: Format Specifiers.
If you want to add a specific content to the string you will need to enclose in with single quotes. Here is an example that shows this.
Using a custom format string
radTimeSpanPicker1.Format = "dd\'days \'hh\'hours \'mm\'minutes \'ss\'seconds \'fff\'milliseconds\'";
radTimeSpanPicker1.Format = "dd'days 'hh'hours 'mm'minutes 'ss'seconds 'fff'milliseconds'"
You can find in the screenshot below the obtained result.