Surely the create new markdown button already does that?
I then presume you are referring to other buttons;
- New Tiddler, Leave it in case you want vanilla tiddlers, use settings > appearance > toolbars
- New Journal $:/core/ui/Buttons/new-journal has its actions in $:/core/ui/Buttons/new-journal
- New Journal here $:/core/ui/Buttons/new-journal-here contains its own actions
In both tiddlers above modify $message="tm-new-tiddler" to include another field=value
- In this case
type="text/markdown"
But I also suggest you enhance this further adding
journal-date=<<now "YYYY0MM0DD0hh0mm0ss000">>’
it provides a reliable value for journal tiddler dated and can be back dated etc…
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