Adding a button to the html editor (textarea) in new PHPFusion 9, xx. Open the file ../includes/html_buttons_include.php and add the code for the new button.
Sample code. Buttons that work for me in pf-9
Ul button
$res .= "<input type='button' type='button' class='btn btn-sm btn-default m-b-10 button' value='ul' title='UL for the list' onclick=\"addText('".$textarea."', '<ul>', '</ul>', '".$formname."');\">\n";
Li button
$res .= "<input type='button' type='button' class='btn btn-sm btn-default m-b-10 button' value='li' title='Li list' onclick=\"addText('".$textarea."', '<li>', '</li>', '".$formname."');\">\n";
Awesome download icon for file links
$res .= "<button type='button' value='download' title='Download Link Style' class='btn btn-sm btn-default m-b-10 button' onclick=\"addText('".$textarea."', '<span class=\'fas fa-download\'>', '</span>', '".$formname."');\"><span class='fas fa-download'></span></button>\n";
DIV block with custom css
$res .= "<button type='button' value='div css' title='Div css for link block download' class='btn btn-sm btn-default m-b-10 button' onclick=\"addText('".$textarea."', '<div class=\'dw-block\'>', '</div>', '".$formname."');\">div css</button>\n";
YouTube button
$res .= "<button type='button' value='YouTube' title='Insert iframe videos from YouTube' class='btn btn-sm btn-default m-b-10 button' onclick=\"addText('".$textarea."', '<iframe width=\'560\' height=\'315\' src=\'https://www.youtube.com/embed/XXXXX?rel=0&showinfo=0', '\' frameborder=\'0\' allowfullscreen></iframe>', '".$formname."');\"><i class='fab fa-youtube'></i></button>\n";
Instead of XXXXX, insert the id of the video. Such a video will not display well on small mobile screens.