Hi Edward,
The input formatting/masking on the web form is not yet supported. Ath this point, you would need to create the form on Survey123 Connect for input format/masking.
To raise the priority of this enhancement request, I'd also suggest you to log this request in ideas.arcgis.com so we can get more users to vote on this request to move the priority further.
Thanks,
Shwu-Jing