The only simple "{0}" format is recognized.
Default format depends on WebSlider.EnableSecondaryValue, WebSlider.Track.FillValueMode and Location properties and it can be "{0}" or "{0}:{1}".
If application needs special format, then it should process ClientEvents.FormatValueLabel.
The "|" character is not supported.
Notes for situation when WebSlider.EnableSecondaryValue is enabled:
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2