Here's an example of the wikitext to add a Comment button to a page:
which results in the following:
Add a Comment
To add a comment, you will need to be logged on to the wiki. If you are logged on, click the button below to add a comment. The comment will be appended to the Discussion page for disposition by the reviewer. <inputbox> type=comment editintro=Comment_Instructions preload=Comment_Preload buttonlabel=Post a Comment on the Discussion Page default=Template talk:Comment hidden=yes </inputbox>
When a logged in user clicks the button, they are given a chance to add a new comment, with the introductory text from Comment Instructions with preloaded text from Comment Preload. Please edit this page to see the template implementation details.