To setup your own numbering scheme you have two options which are both available in the Gear icon (top right) and by clicking at Document Types.
a. Switch numbering from automatic to manual in an existing document in question or to create your own document type with your preset prefix and postfix. You can do
Find the document in question in the list of Document Types and click on its name to open it. You can then uncheck the automatic option and save. 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 Type
Click at Add Document Type and fill in the necessary values. You may need to copy settings from an existing Document Type 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.