Generating Google API JWT via JSONata

{
    'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer', 
    
    /* Custom JSONata-registered JWT creation function ($createSignedJwt is just a pass-through wrapper on top of jsonwebtoken) */
    'assertion': $createSignedJwt({
    
        /* Service account email */
        'iss': '{{client_email}}', 
        
        /* Google scoped service (url format) */
        'scope': $.scope, 
        
        /* The Google Auth URL */
        'aud': 'https://www.googleapis.com/oauth2/v4/token', 
        
        /* Issued At (now) */
        'iat': $round($toMillis($now()) / 1000), 
        
        /* Expiration - Must be exactly 3600 seconds from iat */
        'exp': $round($toMillis($now()) / 1000) + 3600, 
        
        /* Subject email -- for impersonation */
        'sub': $.subject
    }, 
    '{{private_key}}', 
    {
        'algorithm': 'RS256' 
    })
}