Is it possible to use https://github.com/symfony/symfony/blob/7.0/src/Symfony/Component/Mailer/Bridge/Sendgrid/Transport/SendgridApiTransport.php to achieve https://github.com/symfony/symfony/blob/7.0/src/Symfony/Component/Mailer/Bridge/Sendgrid/Transport/SendgridApiTransport.php#L128-L137

From what I see the transport only supports custom_args, but I might be missing something, by docs

{  "from": {
    "name": "myname",
    "email": "[email protected]"
  },
  "personalizations": [
    {
      "to": [
        {
          "email": "[email protected]"
        }
      ],
      "substitutions": {
        "-name-": "Alice"
      }
    },
    {
      "to": [
        {
          "email": "[email protected]"
        }
      ],
      "substitutions": {
        "-name-": "Bob"
      }
    }    
  ],
  "subject": "New mail",
  "content": [
    {
      "type": "text/plain",
      "value": "Hi -name-, lorem ipsum"
    }
  ]
}

should be the proper payload to achieve substitutions

When I hardcode the above payload in the transport, I get the expected output.

I’ve tried to add metadataheaders but that only seems to affect custom_args