You can't set a discrete value for duration, it's limited to short|long (7 or 25 seconds respectively).
By using ShowToastXml
this (and all other) elements can already be set. I'll expose 'duration' and 'scenario' to ShowToastJson
, but since neither of these are recommended for use (ref)
and to keep ShowToast
simple, if you want to use any custom attributes use ShowToastXml