roMessagePort posting message

I think what you're asking here is how to make the app do something?  You can use a timer, when the timer expires have it do something.  Or you can push an input into the app using External Control protocol -
https://sdkdocs.roku.com/display/sdkdoc/Timer

https://sdkdocs.roku.com/display/sdkdoc ... ontrol+API

If you are wanting something to occur when a field in the app changes, use an observer.
https://sdkdocs.roku.com/display/sdkdoc ... ion+Events