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

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.

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.