Microsoft.Azure.WebJobs.Host: Unable to resolve the value for property 'SendGridAttribute.ApiKe y'. Make sure the setting exists and has a valid value.

I am using azure/functions version 4.0.1. Which we recently updated from version 3.
So we removed the function.json and registered our functions like below:

```
import { processor } from "./processor";

import { app, output } from "@azure/functions";

const queueName = config.loadString("AZURE_SERVICEBUS_EMAIL_QUEUE_NAME");
const sendGridApiKey = config.loadString("SENDGRID_API_KEY");

app.serviceBusQueue("EmailSendingFunction", {
connection: "AZURE_SERVICEBUS_CONNECTION_STRING",
queueName,
handler: processor,
return: output.generic({
type: "sendGrid",
apiKey: sendGridApiKey,
}),
});

export default processor;
```

But we get an error:
```
System.Private.CoreLib: Exception while executing function: Functions.EmailSendingFunction. Microsoft.Azure.WebJobs.Host: Unable to resolve the value for property 'SendGridAttribute.ApiKe
y'. Make sure the setting exists and has a valid value.

```

Please provide a resolution as there is no documentation on how the sendGrid output binding works in v4.