You can do this for keys that are part of the flow, or for keys that aren't in the flow but are helpful when submitting data or for a specific computed field. If the key is already part of the flow, skip step 1.
Step 1: Register the Key
Select the Entire Flow by going to the Layers tab of the Left Sidebar and clicking on the "All" tab
Go to the Options Tab on the Right Sidebar and click on the Advanced Dropdown
Type the key you need to register (remember to use snake_case)
If you need to register more than one Key, do it in different lines:
Step 2: Add the Default Values
Also on the Options Tab of the Entire Flow, below the Advanced Dropdown, there's another dropdown called "Default User Data".
Going in there, click on the edit icon and give each key a value, using a JSON format:
Don't forget to save the changes after editing! These buttons will show up where the edit icon was before: