plugin to populate "run custom build" dropdown parameters

I'd like to write a plugin that will populate dropdown prompt parameters.

I looked at similar requests / plugins, but no success:

- Is it possible to create a custom build parameters provider?

- How can I dynamically populate drop downs?

Custom build parameters seems to be populated after "Run Build" clicked in "Run Custom Build" window.

For StaticUIExtensions I can't find appropriate PlaceId  to make "Run Custom Build" page extension.

And I'd like to intercept the event before "Run Custom Build" window shown. 

Is it possible to do this ?

PS: when debug logging enabled I see in the logs only this on "Run Custom Build" window invoke:

DEBUG - jetbrains.spring.web.UrlMapping - Mapping [/runCustomBuild.html] to HandlerExecutionChain 
        with handler [jetbrains.buildServer.controllers.RunCustomBuildController@1f21942] and 1 interceptor