“ Most buttons work with a selection,
make a selection to highlight the text and click the button, or click the button to add the code at the insertion point, then paste or type to replace selection. You can use the code to design new edit buttons, and add them in this page for other editors to use.
For a list of available icons to choose from see MediaWiki edit toolbar. Hiding buttons Existing buttons in the default toolbar can be less useful at Wikisource, or you may want to keep your toolbar to a smaller size. ”

“ Link to a subpage Button link:
What it does: Creates a link from the highlighted text to a subpage from the current page. On the page The Double Dealer, converts the text "Act I" to " [ [/Act I|] ] ", which looks like "/Act I". The actual Wikisource link would be The Double Dealer/Act I.
mw.toolbar.addButton ({ "//upload.wikimedia.org/wikipedia/commons/0/0c/Button_Link_DifferentName.png", "Subpage", ' [ [/', '|] ] ', "Insert link here" }) Relative link Button link:
What it does: Wraps the text in a relative link ( [ [../link location/] ] ) . ”