Credentials are saved in a configuration file which is produced by registration. Example:

# save as your configuration file for later commands
home          =
client_id     = 9**************************************************************3
client_secret = 1**************************************************************c
access_token  = f**************************************************************f
created_at    = 1550501491
scope         = read write follow
token_type    = Bearer

The content is very important for toot operation which reads access_token to be able to post statuses under the identity of the registered account.

Keep this file private!

If you want to cancel the registration of the application, open Settings in your Mastodon, and remove the respective registered application.