Here’s an example that uses $droppable to create a tiddler with either an external link or an iframe, depending upon whether the CTRL key modifier is held down when dropping a URL on the target.
\define make_link() [ext[$(actionTiddler)$]]
\define make_iframe() <iframe src="$(actionTiddler)$" style="width:100%;height:70vh;"></iframe>
\define drop_actions()
<$action-createtiddler $basetitle=<<actionTiddler>>
text={{{ [<modifier>match[ctrl]then<make_iframe>else<make_link>] }}}/>
\end
<$droppable actions=<<drop_actions>>>
<span title="drop URL here to create a bookmark (hold CTRL to create an iframe)">
{{$:/core/images/link}}
</span>
</$droppable>