Besides having a mobile-friendly site, there is nothing you (or Q2A) can specifically do. Google decides mobile-first based on its own factors, probably site popularity.
If you have a responsive site (so the content is the same on desktop and mobile) then mobile first indexing makes no difference anyway. See these resources:
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.
April 20, 2021: Q2A 1.8.6 released!