Hi Nadine,
Survey123 Connect#3.9.120 worked to set the default basemap and web form is also reflected the basemap set in Connect.
In Connect, go to Settings>Map and clcik the basemap chooser(red circle) and pick your preferred one, then publish the survey.
Now go to the web form, the basemap should still honors the basemap you set in Connect.
User can click the basemap chooser(red circle) on the web form if they want to choose other basemap than the default one.
The basemap chooser allows users to overwrite the basemap to their preferred one.