To create a toolbar button you can do:
:text "Print Something"
You should supply image , text and name . This is because the user can customize the toolbar such that one (or all) of these appear, as described in Modifying toolbars.
A toolbar-button cannot be displayed directly. To include it in an interface toolbar, do:
:toolbar-items (list print-button)))
CAPI User Guide and Reference Manual (Unix version) - 25 Feb 2015