Authorization letters are written in order to authorize or approve someone on your behalf to perform an action that should have been done by you. This is a formal document that grants someone else the legal authority to act on your behalf in specific situations.
This can be provided by your organization's letterhead & written in the name of the concerned party for any formal purpose.
For Decatlon you can get the template as well, once you are on the registration page & reach at Upload document step.
