Have fixed your topic subject as made no sense to your post. Regarding your post there's already a feature ticket to implement a param to have the create post input expanded by default.
forge.joomlapolis.com/issues/6771
Also keep in mind all of CB Activity frontend is template driven. You can override how it displays or add custom CSS to bring more attention to the input as needed. The only real limitation on the template usage is HTML structure as the structure needs to be maintained for the jQuery to find its necessary elements. See the below for the default template files.
/components/com_comprofiler/plugin/user/plug_cbactivity/templates