How do I add a separator on a custom Addin toolbar?

there is a "Search" function in the API help ;-)

SldWorks::AddToolbarCommand2 "To create a separator, all of the method's string parameters (ButtonCallback, ButtonEnableMethod, ToolTip, and HintString) must be blank strings. A bitmap button must still be defined in the add-in's resources, but it will not be used."