def request_cert(session, domain_name, validation_domain):
    """Requests a certificate in the AWS Certificate Manager for the domain name

    Args:
        session (Session|None) : Boto3 session used to communicate with AWS CertManager
                                 If session is None no action is performed
        domain_name (string) : domain name the certificate is being requested for
        validation_domain (string) : domain suffix that request validation email
                                     will be sent to.

    Returns:
        (dict|None) : Dictionary with the "CertificateArn" key containing the new
                      certificate's ARN or None if the session is None
    """
    if session is None:
        return None

    client = session.client('acm')
    validation_options = [
        {
            'DomainName': domain_name,
            'ValidationDomain': validation_domain
        },
    ]
    response = client.request_certificate(DomainName=domain_name,
                                          DomainValidationOptions=validation_options)
    return response