The first thing you'll need to do to connect your Withings account is to go on "connect apps" from your desktop or click on this link.
This will trigger a new window to open where you'll be asked to login from the button on the top right corner of your screen. This will open a pop-up to ask you the permission to create a new stream dedicated to receiving your Withings data. If you decide to reject your data won't be uploaded and the Withings stream won't be created.
Once you accepted you will get a last confirmation screen to tell you that all new data will now be imported to your Pryv account in its dedicated stream.