Assuming you don't want the "A" keypress to get passed through, you'll need to use a command that triggers off of that key and has the "Do not allow key to be passed through" option enabled
So in total, you'd have three commands: One triggered by the "A" key, another for "On", and then the last one for "Off"
The latter two commands would just set a Boolean variable to either true or false.
The command triggered by the keypress would then check the value of that variable, and if it's true, perform the function you're describing, whereas if the variable value is not true, it'll instead press the "A" key using a keypress action.
To preserve the main functionality of the regular "A" key while the variable value is not true, you'd want to use a keypress action that presses the key, followed by a while loop that checks whether the key is still pressed, and then a keypress action that releases the key again.
Note that this will mean that holding down the "A" key does not produce multiple "a" characters, as virtual keypresses do not trigger that system.