1). Edit asg-core.cdd and add the custom headers in the CORS section.
<property name="be.http.filter.cors.param.cors.allowed.headers" value="databaseName, Origin, Accept, apikey, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers"/>
Here the header "databaseName" is a custom header.
2). Restart the gateways.
ASG-6174 is available to make this more seamless and is scheduled for release 2.1.1HF1. This is subject to change.