I have implemented the draw feature on map as mentioned in this post and it works like charm on Mobile as well desktop Survey123.
If I try to consume it in survey123 webapp then, I cannot see Map as a canvas instead it asks for image to be uploaded and then also complete options are not appearing unlike in Mobile and desktop use case.
However, In mobile and desktop survey123 once user clicks on pencil icon as highlighted in below image it opens the webmap where user can select the map extent and then draw on top of it.
Hi. Markup is not supported in the web app.