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Whenever I try to access serverless lambda function via POST through the browser I get the error

Response to preflight request doesn't pass access control check: No >'Access-Control-Allow-Origin' header is present on the requested resource.

When it is a /GET it works fine I have read it is because it is not sending pre flight request. When I change it to POST this is when it fails.

The command I am running:

sam local start-api

And my template.yaml is:

...

Resources:
 PropertiesFunction:
 Type: AWS::Serverless::Function
 Properties:
 CodeUri: target/service-0.0.1-SNAPSHOT.jar
 Handler: com.aws.PropertiesHandler::handleRequest
 Runtime: java8
 Events:
 PropertiesApi:
 Type: Api
 Properties:
 Path: /properties
 Method: post

...

How can I enable CORS on these endpoints?

You should be able to get around this for local testing by explicitly adding the following headers to your response, in your handler function:

 "Access-Control-Allow-Origin": "*"

You can use an environment variable that only adds the headers if you're running locally.

Here's a simple Python example from a handler function I have:

 if not all(field in values for field in required_fields):
 response = 
 'statusCode': 400,
 'body': json.dumps(
 
 'message': 'Required data missing from request body'
 
 ) 
 
 # explicitly add CORs headers for local testing
 response['headers'] = "Access-Control-Allow-Origin": "*"
 return response

This is only suitable for local testing, when you deploy to an API Gateway CORs is handled by configuration on the API Gateway.

This solution worked for me until I also added authorization to the API, with a Cognito user pool, which I'm currently trying to work through.

Here's a similar post on the issue:
How to Enable CORS for an AWS API Gateway Resource

Here's reference to it in the official AWS documentation.