There is no "tm-open-all-tiddlers"
message. See the list : https://tiddlywiki.com/#Messages
What you can do is use a list widget inside of a button widget, and loop trough a list of tiddler to open, then use an action-navigate widget to open each one. Assuming your list is in the list field of a tiddler named "DefaultTiddlers" :
<$button>
<$list filter="[list[DefaultTiddlers]]">
<$action-navigate $to=<<tiddler>>/>
</$list>
Open all
</$button>
You can also use a use a link to do that :
<a href="#:[list[DefaultTiddlers]]">
Open all
</a> But this will close tiddlers that already are in the story. To prevent that, use this :
<a href="#:[list[$:/StoryList]] [list[DefaultTiddlers]]">Click to add the default tiddlers bellow the story</a>or
<a href="#:[list[DefaultTiddlers]]
[list[$:/StoryList]]">
Click to add the default tiddlers above the story
</a>If you want your link to look like a button :
<a href="#:[list[DefaultTiddlers]]
[list[$:/StoryList]]"><$button>
Click to add the default tiddlers above the story</$button>
</a>
This is shorter than a list widget, but this will modify the url.