Press Key Trigger: Capital Letters

// Function to set the Storyline variable
function setStorylineVariable(variableName, value) {
    GetPlayer().SetVar(variableName, value);
}

// Check if listeners are already added
if (!window.keyListenersAdded) {
    // Combined event listener for keydown and keyup
    document.addEventListener('keydown', function(event) {
        if (event.key === 'A' || event.key === 'a') {
            setStorylineVariable('isKeyAPressed', true);
        }
    });

    document.addEventListener('keyup', function(event) {
        if (event.key === 'A' || event.key === 'a') {
            setStorylineVariable('isKeyAPressed', false);
        }
    });

    // Mark listeners as added
    window.keyListenersAdded = true;
}

Personal approach:

Introduce a new T/F variable with this JS script: When key A is pressed, set it to true; When released, set it to false.

The new trigger will be: When variable changes, if isKeyAPressed is true, [exec function here].