@marnall is correct. I don't believe that Dashboard Studio supports checkbox but aside from look and feel, multiselect provides all functionality that checkbox does. You can also ask in this dedicated forum Dashboards & Visualizations.
I understand look and feel. You can search https://ideas.splunk.com/ to see if someone is asking for this parity; if not, you can submit an idea for it.
Unless I am mistaken, the multiselect input should provide the checkbox functionality you desire. Does multiselect work for your use case?