myTimeSpan.ToString(@"hh\:mm")
Custom TimeSpan Format Strings
The custom TimeSpan format specifiers do not include placeholder
separator symbols, such as the symbols that separate days from hours,
hours from minutes, or seconds from fractional seconds. Instead, these
symbols must be included in the custom format string as string
literals. For example, “dd.hh\:mm” defines a period (.) as the
separator between days and hours, and a colon (:) as the separator
between hours and minutes.