For some reason, we don't actually expose a way to set the encoding of an async connection in the API. It would technically be possible to use AsyncMysqlClient::adoptConnection to create the connection via MySQLi and set the encoding before passing it to the async system, but I wouldn't recommend doing it that way.
Instead, as suggested here, you could set the encoding of the connection explicitly via SQL with:
SET collation_connection = utf8mb4_unicode_ci;
SET NAMES utf8;
If you have access to the configuration of the MySQL server itself, you can use init_connect
to have the statements run at the start of every, non-root, connection:
[mysqld]
init_connect='SET collation_connection = utf8mb4_unicode_ci; SET NAMES utf8;'
default-character-set=utf8
character-set-server=utf8
collation-server=utf8mb4_unicode_ci