Default save folder
Update settings to make the app functional. "Save to a default folder" is so incredibly basic I can't believe you're willing to put your name on the program without the feature.
Being unable to save files in my prefered Dropbox folder pushes me away from Microsoft and Office Lens particularly.
Sofiane Ayoute commented
Actually I've just found that you can do it. You only have to do it VIA the OneDrive app, go to the folder of your choice and select the "+" then "scan". It's apparently the same way for Google Drive.
For each of the destinations that lens can save to on each one you should be able to select a default e.g. on OneNote every time you save an item you have to choose your default folder unless you want it to go to quick notes which it does if you don't alter it each time