In the MRSS setup you can't add code breaks. When using MRSS Roku will play an ad every 7 minutes. Adding code breaks can only be accomplished using a JSON feed.
Ad policy and frequency rules Ads are not shown for the first 15 minutes of content after a user installs your channel A pre-roll ad is shown for every 7 minutes of content. E.g.: if a video is 22 minutes long, three pre-roll ads will be shown.
Ad types When using Direct Publisher, the only available ad position options are Pre-roll and Mid-roll; post-roll ads can not be served. Only our JSON feed specifications support mid-roll ads. Use the "adBreaks" field to set break points for mid-roll ads in your content.