Hi,
Your button must have an onclick pointing to a function in your custom js – like you have it now, the custom js is running when you open the form and not when you click the button.
Use this button HTML code:
<input type="button" onclick="showCommentBox();return false;" value="Show commentbox">
And this Custom JS:
function showCommentBox(){
spjs.dffs.triggerRule(["CommentBox"]);
}
Alexander