To set up your own numbering scheme you have two options which are both available on the menu and by clicking at Document Templates.
a. Switch numbering from automatic to manual in an existing document in question
Find the document in question in the list of Document Templates and click on its name to open it. You can then uncheck the automatic option and Update. Following that, you will need to enter the document number manually. Note that once you switch from automatic to manual you cannot go back to automatic.
b. Create a new Document Template
Click on New Document Template and fill in the necessary values. You may need to copy settings from an existing Document Template if you're not sure about what configuration to use. In any case, make sure that in the numbering scheme field you keep automatic and enter your preferred prefix and postfix.
Or login to your account.