The shift+alt+L shortcut is handled by the tiddler $:/core/ui/EditorToolbar/linkify. Its text looks like this:
<$action-sendmessage
$message="tm-edit-text-operation"
$param="wrap-selection"
prefix="[["
suffix="]]"
/>
It wraps your selection (or your caret) with a prefix [[ and a suffix ]]. So to insert your macro, you would replace the prefix and suffix attributes and other fields.
<$action-sendmessage
$message="tm-edit-text-operation"
$param="wrap-selection"
prefix='<<ML "'
suffix='">>'
/>
I suggest to make a new shortcut though. You could clone the linkify tiddler instead. Then you need to bind it to a shortcut. There’s a guide at tiddlywiki.com. Check the “How to create editor toolbar buttons” and “How to create keyboard shortcuts”, and also: “WidgetMessage: tm-edit-text-operation” for more on how the editor toolbar buttons work.
Have a great day!