Hi Sebastian! Yes, it's possible to write an open hi-hat. Once you've added a note to the hi-hat instrument, tap on the diamond multiple times to change the articulations. The hi-hat currently supports the following articulations, in this order:

I'm working on some videos to show how to add these kinds of articulations. Almost all instruments support some articulation, so please experiment with the snare drum and other drums and cymbals to see what is available. Not all articulations are included yet - as you'll see, I have some open feature requests to add flams and other articulations. Please let me know if you'd like to add anything else. Thanks for supporting Beat Note and taking the time to ask a question!