How to define a service for mailer with a specific transport

I am using Monolog with HTML email formatter to send some alerts.

monolog:
    # ....
    symfony_mailer:
        type:           symfony_mailer
        from_email:     "%admin_email%"
        to_email:       "%admin_email%"
        subject:        "A problem occurred"
        level:          info
        content_type:   text/html
        formatter:      monolog.formatter.html

I want sending email with a specific email account defined as transport “monolog”

# config/packages/mailer.yaml
framework:
    mailer:
        transports:
            main: '%env(MAILER_DSN)%'
            monolog: '%env(MAILER_DSN_IMPORTANT)%'

For that, I can specify a service to use in the monolog config like mailer: 'monolog_mailer_transport' but I am not able to define the service to use the “monolog” transport.

service:
    # ....
    # NOT WORKING : HOW TO DEFINE THE SERVICE TO USE THE TRANSPORT 'MONOLOG' ?
    monolog_mailer_transport:
        class: SymfonyComponentMailerTransportTransportInterface
        factory: ['@mailer.transports', 'get']
        arguments: ['monolog']