ToolBox native messaging api documentation anywhere?

The toolbox browser extension is really cool. I'd like to build on it but the use I am looking for is not a `checkout` call. I notice the browser extension does this call:

jetbrains://idea/checkout/git?checkout.repo=git@github.com:JetBrains/toolbox-browser-extension.git&idea.required.plugins.id=Git4Idea 

Is there anyway I can read about what else is available? For example, is there a way to simply open a project at a given directory?

Thanks.