How to add .sig key to the pacman database?

I want to add key, have .sig file. How do I add it?

What is the signature file for?

You’ll most likely want to import the key to your user keyring, not the system keyring. See GnuPG - ArchWiki

If it’s a third-party repo, see pacman - ArchWiki

:information_source: Note that third-party repos are neither supported by Arch nor Manjaro.

I want to install thunderbird-appmenu binary. Usually, I was able to do it, but now pamac complains that the key is missing. In the same place where binary is, there is also .sig file.

The binary is from AUR package maintainer and is compiled on SUSE servers. Basically, the binary is accessibly quicker than the AUR package PKGFILE is updated.