On January 17, the Maryland Secretary of State announced which presidential primary candidates are discussed in the news media, and which will be on the presidential primary ballots automatically.
For the Republican ballot, he chose President Donald Trump and William Weld. See the list for both major parties here.
Candidates not chosen may still get on the ballot if they submit 3,200 signatures of party members by January 24. The petition must include 400 valid signatures from each of the eight U.S. House districts. Thanks to J. Bradley Jansen for the link.