The Notepad++ developer doesn’t actually implement any builtin languages himself: he uses a library called “Lexilla” for doing the syntax highlighting. As a result, if you wanted that added natively into Notepad++, you would have to go make an official feature request at the Lexilla Project “Issues” page, then wait for them to decide whether to implement it or not (doubtful; from what I remember, their response to requests for a new language is usually “we’ll include it if someone will write it and create a PR for us”, unless it’s just a slight variant of an existing language); after Lexilla creates or merges the new language lexer, then you have to wait for Notepad++ to update its Lexilla copy (nowadays, it tends to do that any time the developer has noticed a new Scintilla/Lexilla release)

In general, the expectation is that for languages that aren’t in the list of 80+ languages included with Notepad++, the first line of defense is for the user to create a User Defined Language (“UDL”), with first checking the User Defined Language Collection and its list of contributed UDL definitions to see if someone else already has created it and made it public – I do see “DAX” with a description that confirms it’s Power BI, so at least that one already exists for you. You can follow the instructions in the Collection for how to install a UDL from the Collection.