If you have a new weight, then Add row to Google Drive spreadsheet
If
When I have a new weight
This trigger fires every time you have a new weight from your Withings scale.
Then
Add row to spreadsheet
This action will add a single row to the bottom of the first worksheet of a spreadsheet you specify. Note: a new spreadsheet is created after 2000 rows.
Explore more great ways to automate Google Sheets and Withings