Sleeknote assigns you a Customer ID - In addition, an ID for each note is also generated.
At the Sleeknote Dashboard click Tracking Code.
Copy the ID as shown in the image above (Use only the digits, in this case, 2571)
Select your Flipbook
Go to Settings > Integration > Sleeknote
Paste or enter the ID into the Sleeknote Customer ID field
Press Save changes.