Try as below in get_buttons or modify_buttons method of overview page
data: ls_button type crmt_thtmlb_button_ext.
ls_button-text = 'Google'.
ls_button-on_client_click = 'javascript:window.open("http://www.google.com","test",width=100,height=100);'.
ls_button-enabled = 'X'.
you can prepare the dynamic javascript string