The document has to be uploaded as a "MS Letter to request an extension of the deadline" in the Documents section of the Accounts.
For this you need to create the Accounts dossier for the given accounting year first!
You can refer to the instructions in section Create accounts and in section Documents of the quick guides IGJ ETC , IPACB , EMFF , FEAD , AMIF ISF , EAFRD EAGF .