You should place the asc file in /etc/apt/trusted.gpg.d/ (or you can use the apt-key add command but it is deprecated).
This is the apt-key add command related manual entry:
add filename (deprecated)
Add a new key to the list of trusted keys. The key is read from the filename given with the parameter filename or if the
filename is - from standard input.
It is critical that keys added manually via apt-key are verified to belong to the owner of the repositories they claim
to be for otherwise the apt-secure(8) infrastructure is completely undermined.
Note: Instead of using this command a keyring should be placed directly in the /etc/apt/trusted.gpg.d/ directory with a
descriptive name and either "gpg" or "asc" as file extension.