Add Source Button to h5pEditor

Can anyone point a way to do this? By following the instructions at https://h5p.org/adding-text-editor-buttons I have built a module with an after_build hook and manage to add tags and buttons to the h5peditor tool bar, but I cannot seem the add the Source (i.e. sourcearea) button plugin? I ahve downloaded the plugin and the js to load it is being run but the H5PEditor.HtmlAddons.text.sourcearea function id never breing called? Any ideas?

Thank you