There are couple of ways you can run composer with Plesk Shell. My favorite one is to add php to your PATH variable, and it will automatically add the composer as well. You may follow through the following to modify your shell path variable to use Plesk PHP:
Once done, now you may run composer command and it shall work:
[elastic-keldysh@pl1 ~]$ composer -V Composer version 1.10.5 2020-04-10 11:44:22 [elastic-keldysh@pl1 ~]$
The other way, is to directly use the composer.phar given from Plesk 9.0 library. This file is available under the following location:
/usr/lib64/plesk-9.0/composer.phar
So, you may run this using the following:
[elastic-keldysh@pl1 ~]$ /opt/plesk/php/7.4/bin/php /usr/lib64/plesk-9.0/composer.phar -V Composer version 1.10.13 2020-09-09 11:46:34
This should work too. You may choose any, and it shall work for you. Good luck.