1- Go to Account -> Developers and click on Create New App. Here's a link that'll take you there directly: https://app.put.io/account/api/apps/new.
2- Now fill in the boxes. You can name the app however you like.
3- After saving, click on the key icon next to the app you just created. It'll take you to the Secrets page and here, you can copy the token created for you.