plugin function call onClick

Hey.

I’ve been having a go at creating a custom plugin (simplified code below).

I have my plugin file (counter.php) which has 2 function in it called

displayCount() {
    // output count and a link to add 1
    echo '<a href="XXX">Add One</a>';
}

AND

addOne() {
    $count = $count + 1;
}

My question is what to I replace XXX with or how do I call addOne function from my post page?

Thanks