Community of the Android app Boost for Lemmy
AFAIK, it's complicated.
Check out this app which works for Sync so I'm sure could also support Boost in the future.
It requires adb (or root) to initially configure it but it's an easy one-line adb command and from then on it updates a list of supported instances and automatically handles intent redirects to your preferred app. They also have one for Mastodon.
You need a separate app to handle it, look for "Lemmy Redirect" on fdroid. I haven't set mine up yet, but it looks like it supports Boost now as well
It's already possible, no additional apps required, but you have to enable it manually because Boost doesn't own all those domains.
Go to Boost's app info -> Open by default -> Add link, then choose the instances that you want to use Boost to open.
It's close, but not as elegant as it was in Boost for reddit.
Now the links open in Boost, but Boost is reduced to an android webview around the interface of my instance, instead of rendering the interface (loading inbox, displaying comments, etc) in Boost like it used to.
Yeah, looks like it only works if the link is to a post or community, but not the front page of an instance.