I add the Spectrum add to my home page, but when I switch to guest mode so that my guests can use it, it’s not there. How can I make it show up in guest mode so my guests don’t have to add it themselves
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Sorry, but I don't think it's possible. All you can do is leave instructions for your guests for how to add it.
Well then how do the other pre-loaded apps show up?
Well then how do the other pre-loaded apps show up in guest mode?
Roku has a "default" group of apps that install automatically, and that's likely what you get in guest mode. But as @renojim mentioned, Roku does not provide any way to preconfigure what is available for a device in guest mode. It's been requested for some time, but so far nothing has come from it.