It seems like this:
[root@iz2ze5bh2113hkwcyk6afnz ****]# composer require xengine/flarum-ext-signature
[InvalidArgumentException]
Could not find a version of package xengine/flarum-ext-signature matching your minimum-stability (beta). Require it with an explicit version constraint allowing its desired stability.
require [--dev] [--prefer-source] [--prefer-dist] [--no-progress] [--no-suggest] [--no-update] [--no-scripts] [--update-no-dev] [--update-with-dependencies] [--update-with-all-dependencies] [--ignore-platform-reqs] [--prefer-stable] [--prefer-lowest] [--sort-packages] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--] [<packages>]...
[root@iz2ze5bh2113hkwcyk6afnz ****]#