It does not look like Newsmax has updated their OTT channels/apps to support Newsmax+ yet. Will need to wait for them to update or release a new channel/app for the Roku platform.
I noticed the login screen for the Newsmax+ appears when trying to watch online at their site. Here you can login/signin/signup. https://www.newsmaxtv.com/ You could then cast to your TV or through Roku, or use a HDMI cable from your laptop directly to your TV. (no Roku required)
I like to use Web Video Caster for casting online content to my Roku devices. Install the companion receiver on your Roku device/RokuTV, and then the Web Video Caster app on your phone or mobile device. https://channelstore.roku.com/details/dd37e6d096d3197a9578787890118e33/web-video-caster-receiver
I noticed when I installed the Newsmax app on my phone, it automatically had Newsmax+, Newsmax2, etc and played LiveTV fine without signing in. (did not create an account for Newsmax). Did not seem to be a preview (timed), though this is likely to change.
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Almost seems that Newsmax kind of forgot their agreement went into effect today. Either that, or the lack of having an available OTT channel/app ready to go for Roku and other streaming services shows users their newly found priority.