With Play Framework, you can override configuration keys by specifying them as Java System properties. Example:
activator run -Dsome.key="some.value"
But if you have a String List key, you cannot override it like in the configuration file. Example using the trusted proxies configuration:
activator run -Dplay.http.forwarded.trustedProxies=\["127.0.0.0/8","10.0.0.0/8"\]
You will have this runtime error:
Configuration error\[system properties: play.http.forwarded.trustedProxies has type STRING rather than LIST\]
To avoid this, you have to specify each entry of the list using the entry index:
activator run -Dplay.http.forwarded.trustedProxies.0=127.0.0.0/8 -Dplay.http.forwarded.trustedProxies.1=10.0.0.0/8
Note that there are no double quotes around the values. I encountered some issues when using quotes, and it works fine without them.