Cannot configure Ebean read replica with Play configuration

I have been successfully using a read replica with Play Framework 2.7 for years. I am upgrading to Play Framework 2.9 and Ebean 15.1.0 with Play Ebean Plugin 7.3.0.

I have tried two different configurations and with both of them, the default database is recognized and used, but the read replica is not. Here are my two attempts (application.conf):

db.default.driver=org.postgres.Driver
db.default.url='xxxx
db.default.readOnlyUrl='xxx`

db.default.driver=org.postgres.Driver
db.default.url='xxxx

db.readonly.driver=org.postgres.Driver
db.readonly.url='xxxx'
db.readonly.readOnlyDatabase=true

Here is a link to the documentation that shows the 'readOnlyUrl' property in the configuration.
https://github.com/ebean-orm/ebean-datasource/blob/master/ebean-datasource-api/src/main/java/io/ebean/datasource/DataSourceConfig.java#L719

Please advise on the proper way to configure a read replica.

Start with the application.conf examples in the issue and the referenced ebean-datasource DataSourceConfig.java readOnlyUrl definition. Trace how Play Ebean Plugin 7.3.0 maps Play settings to Ebean 15.1.0 datasource configuration, then verify a supported configuration makes both the default database and read replica usable.