RSS Feed/News Create an option without showing it on the admin options page

[ENXF NET]

Is there a way to create an option that will be used to store an internal API key that the user doesn't need to see at all, even not with the "Show advanced option" ?
I've tried it with a custom callback which returns an empty string, but that's showing an error because of the following template code:

Code:

        <xf:if is="$includeHtml is empty">
            <xf:formrow rowclass="{$rowClass}">
                <div class="blockMessage blockMessage--error blockMessage--iconic">...

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