ROKU should be able to answer this simple question. If they can't, they should find out if it's possible. If not, it should be possible because it's what we the customers want. Thank you.
If you are talking about within the Spectrum/Cable app, when you hit the back arrow to bring up the main menu, Live/Guide/Library, etc, there is a little cog wheel in the lower left, in here you will find a preference for a startup channel option.
We don't have Spectrum in Iowa, and I don't have cable. I have a Roku TV and several Roku adapters. This is a Roku forum, not a cable service forum. My live channel is through Paramount+, which Roku and my Google speaker can't seem to find verbally.