The following key bindings require Composer 4.2.5 or above. (Or they can be implemented for any version in the Mac OS KeyBindings file).

To use them, type the desired label for a footnote, leaving the cursor before the closing ]. For example:

Text.[^foot|]

Trigger the first command and you will end up with:

Text.[^foot]

[^foot]: |

Type the desired footnote and trigger the second command to return to your initial position in the document.

(This only works for footnote labels that consist of a single word)

[{
        "key": "f",
        "modifiers": ["cmd", "shift"],
        "action": [
            "setMark:",
            "moveWordBackwardAndModifySelection:",
            "copy:",
            "moveToEndOfDocument:",
            "insertText:", "\n\n[^",
            "paste:",
            "insertText:", "]: ",
            "moveToEndOfDocument:"
        ]
    },
    {
        "key": "f",
        "modifiers": ["cmd", "shift", "opt"],
        "action": [
            "swapWithMark:",
            "moveForward:"
        ]
    }
]