To create a toolbar button you can do:
(setf print-button (make-instance 'toolbar-button :image :std-print :text "Print Something" :name :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 9.6 Modifying toolbars.
A toolbar-button cannot be displayed directly. To include it in an interface toolbar, do:
(display (make-instance 'interface :toolbar-items (list print-button)))
CAPI User Guide and Reference Manual (Unix version) - 01 Dec 2021 19:32:36