How should I pull my passwords from AWS Secret Manager into spring?

Question

I'd like to retrieve my database password from the AWS Secret Manager based off of a key name set in my properties files. It doesn't seem like I can read properties from the application.yml file like I had hoped, however. In the code below, my.project.aws-secret and my.project.region are both retrieved as null.

Am I wrong to expect those properties to be retrievable at this point? And is this the right approach for achieving this, or is there an easier way?

Here is the code. I am currently trying to do this using an ApplicationListener, but I have seen the same problem implementing this as an EnvironmentPostProcessor as well:

ApplicationListener

public class DatasourceAwsProperties implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {

    @Override
    public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        Properties props = new Properties();

        String datasourcePassword = environment.getProperty("my.project.aws-secret"); // why is this null?
        String awsRegion = environment.getProperty("my.project.region"); // why is this null?

        String secret = getSecret(datasourcePassword, awsRegion);

        MapPropertySource newProperties = new MapPropertySource("newProperties", ImmutableMap.of("spring.datasource.password", secret));

        environment.getPropertySources().addLast(newProperties);
    }

    public static String getSecret(String secretName, String region) {
        // gets secret from aws
}

application.yml

lots:
  of:
    properties: ...

my:
  project:
    aws-secret: the-name-of-the-secret-in-aws
    region: us-east-1