Using Function Keys

MacOS interferes with direct access to function keys, which you want to use for starting message transmission by tapping a single key. To avoid holding the fn key along with a function key:

Open Keyboard system preferences
Choose the Keyboard tab
Check the Use all F1, F2, etc, keys as standard function keys option

To prevent system applications from intercepting function keys:

Open Keyboard system preferences
Choose the Shortcuts tab
Select Mission Control and uncheck the items that use F11 and F12
Select Services and uncheck any items that use function keys