How can I make aws amplify uses iam authentication to query from appsync?

I am trying to build a graphql client to query/mutation on AWS appsync. I come up with the library aws-amplify. My appsync uses IAM authentication method. I created an IAM user in my AWS account and I have the key and secret. But I don't know how to authenticate the amplify to use the iam authentication. I have searched AWS doc and it only mentions using cognito user pool which I don't want.

Below is my code. Is there any way I can specify the IAM credentials in the code?

const config: ResourcesConfig = {
  API: {
    GraphQL: {
      endpoint: 'https://xxx.appsync-api.ap-southeast-2.amazonaws.com/graphql',
      region: 'ap-southeast-2',
      defaultAuthMode: 'iam',
    },
  },
};

Amplify.configure(config);

const client = generateClient();

client.graphql({
  query: gql`
    query getUser($id: ID!) {
      getUser(id: $id) {
        id
        name
      }
    }
  `,
...
});