You really can't anymore because Roku banned all private channels back in early 2022. There used to be a pornhub app and some others but now they are gone.
What you need to do is get a Fire TV Stick because there are many adult apps you can sideload, some free and others you can subscribe to like Adult Time for example, which has a very good library selection of adult videos. The Fire TV devices also have web browsers you can add, so you can go to adult websites as well.
The only way you can view anything adult on Roku presently would be if you stream your own media files from you computer over the Plex or Empty apps. Or put files onto a USB stick and plug it into your Roku if your Roku player has a USB port. Roku is very limited and you can only use the official apps in the store.
How do I get On the channel.
On what channel? It's already been said that no adult content is available on Roku devices. You could screen mirror a computer to the Roku, that's the only way to get such content.
This is absolutely false. Roku hosts Many commercial channels that are slap full of soft core porn grade movies. Some of the movies being broadcast today thru streaming channels would be consider rated X 20 years ago. Occasionally I accidentally find French adult-themed movies that contain actual graphic porn shots. One only has to name 1 out of 200 random taboo-themed movies to prove that Roku doesn't block rated X grade movies... I can name at least 20 movies off the top of my head out of a much longer list of unacceptable movies that are scattered throughout all of the streaming channels. Roku (the channel) may or may not host these movies but Roku (the platform) does. I can go even further by saying that the streaming world is moving more towards normalizing porn into mainstream movies. So don't try to make it sound like Roku doesn't allow adult material when it's literally everywhere.