Hi Sarah,
Input mask is not supported yet with web form and there is currently an existing enhancement request for this:
ENH-000120355 Include the ability to utilize Input Masks into the Survey123 Web Application
I encourage you to contact Esri Support. Our Support team will assign an official enhancement number for your records. Similar requests from other customers can then be attached to the same enhancement request, which helps us assess demand for the enhancement and prioritize it accordingly.
Thanks,