Error while passing JSON value in Retool Embeds

I have created a new user attribute called external_data and I am trying to create a Retool embed link.

The type of external_data is JSON

The following is my payload

{
    "landingPageUuid": "xxxxxxxx-xxxx-xxxx-xxxx-2737dd6af548",
    "externalIdentifier": "Jon Doe",
    "groupIds": ["12345"],
    "metadata": {
        "mode": "light",
        "org_id": "1",
        "external_data": {
            "user_id": "test_id"
        }
    }
}

I get error for external_data

{
"statusCode": 422,
"error": "Unprocessable Entity",
"message": "Value is not valid JSON",
"data": null
}

What would be a valid JSON for external_data?