Below seems to be the care minimum code to enable the closed captioning. I tested a track with a SRT file and it worked, so it appears Roku also supports SRT files.
videoContent = createObject("RoSGNode", "ContentNode")
videoContent.SubtitleConfig = {
ShowSubtitle: 1,
TrackName: "URL2ClosedCaptioningFile"
}
m.video.content = videoContent
m.video.globalCaptionMode = "On"
The additional question I have is that I noticed in some apps, when I click the options button, a closed captioning menu appears. Is this a menu that the app developer created or is this a Roku-made menu that the developer can enable?