There are many people trying to make use of disneyplus.com sign in/register to start the process of activating it through the Roku device and if you're one of those guys then don't worry since this is how it could be accomplished.
Go to the channel store by using the Roku remote.
Find the Disney Plus from the main menu.
Click the Install button and download it.
Go to the channel for Disney Plus.
Then , an activation number will appear on the screen.