The list of pre-made buttons doesn't contain everything (that'd be a lot of commands!). Some commands need to either be made via the button editor or copied from the stock toolbars (whichever is easiest).
The secure screenshot menu items run these commands in the stock toolbars:
Lister:
Clipboard SCREENSHOT=secure
Whole screen:
Clipboard SCREENSHOT=secure,all
Or you can paste this on to your menu:
<?xml version="1.0"?>
<button backcol="none" display="both" label_pos="right" separate="yes" textcol="none" type="menu">
<label>&Secure Screenshot</label>
<icon1>#screenshot</icon1>
<button backcol="none" display="label" label_pos="right" textcol="none">
<label>&This Lister</label>
<tip>Copy a secure screenshot of this Lister to the clipboard.</tip>
<function type="normal">
<instruction>Clipboard SCREENSHOT=secure</instruction>
</function>
</button>
<button backcol="none" display="label" label_pos="right" textcol="none">
<label>&Whole Screen</label>
<tip>Copy a secure screenshot of the whole screen to the clipboard.</tip>
<function type="normal">
<instruction>Clipboard SCREENSHOT=secure,all</instruction>
</function>
</button>
</button>