@xfok,

1. Download and Install "Code Snippets" Plugin for WordPress

2. Open Dashboard > Snippets > Add New 

3. Write a Title

4. Add the Below Code, Check the "Only run on site front-end" Checkbox and press Save Changes and Activate

add_action( 'wp_footer', function () { ?>
<script>

jQuery(document).ready(function ($) {
    if (typeof wpforo_editor !== 'undefined') {
        $('#wpforo-wrap').on('click', '.wpforo-qa-comment, .wpforo-reply.wpf-action.wpforo_layout_4', function () {
			setTimeout(function(){
				wpforo_editor.set_content('', $('.wpforo-post-form').data('textareaid'));
			}, 100);
        });
		$('#wpforo-wrap').on('click', ".wpforo-reply:not(.wpforo_layout_4)", function(){
			setTimeout(function(){
           		 wpforo_editor.set_content('', wpforo_editor.get_main());
			}, 100);
        });
		
    }
});

</script>
<?php } );