I installed the latest version of survey123 (Windows x86) on a win10 tablet but My Surveys\Maps folder doesn't exist upon installation. I added the folder with the tpk but it's not showing in the basemap choices.
When you say that you "added the folder", are you referring to theC:\Users\UserName\ArcGIS\My Surveys\Maps folder?
What happens when you attempt step 3 listed in the following linked resource page?
Prepare basemaps for offline use—Survey123 for ArcGIS | ArcGIS
n the Survey123 field app, select Menu > Settings > Map and browse to your ~\ArcGIS\My Surveys\Maps Map Library Folder.
Any survey that has includeLibrary set to true (for additional details, see the last section in this topic) will now include any maps stored in this folder in the list of available maps.
Retrieving data ...