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:
https://webmasters.googleblog.com/2018/03/rolling-out-mobile-first-indexing.html
https://moz.com/blog/mobile-first-indexing-seo
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