One of the goals with this extension for me personally was to find a way to talk to Roll20 "from the outside". This browser extension comes with an optional "native host" that opens a local web server while a Roll20 tab is open, which you can use to send messages to Roll20.
This feature is fully optional, and has to be enabled manually, by registering a "native host" with the ID de.sirs0ri.deck20.
The full documentation can be found here, for Windows you'd have to register the path to the src-native-bridge/manifest.json file (not the manifest file found at src-bex/manifest.json!) at the following registry key:
HKEY_CURRENT_USER\SOFTWARE\Google\Chrome\NativeMessagingHosts\de.sirs0ri.deck20
The web server will automatically start when the browser extension connects to a Roll20 tab, and close when the last Roll20 tab is closed. It is available at localhost:8000 and will accept POST requests. As an example, I'm using the following JSON-payload to roll a d20 from my Stream Deck (via the API Request plugin):
{
"command": "send-message",
"data": {
"msg": "/r 1d20"
}
}