You can use an open source program called "Remoku". It is available as an extension for some browsers or you can just go to http://remoku.tv/. (on the top of the on-screen remote you will see a little keyboard icon).
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Basically, when you launch Remoku, go to Settings on the on-screen remote and type in your IP address of your Roku device (found from Roku's Settings/System/About). Remoku will then connect to your Roku device. (look for the "manual add" box, type in the Roku device IP address, and then press the "+" button).
I generally use the Roku remote to navigate around and just use the keyboard in Remoku, but the on-screen remote of Remoku can navigate around as well. An added benefit is that you can use the arrow keys of your keyboard to navigate around different menus (like within search results, Home Screen, etc. Just press the "tab" button on your keyboard to toggle between the arrows and the keyboard typing). Remoku also supports copy/paste which comes in handy for long or complicated passwords. (and those that use password managers).
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Remoku has been around for awhile and is the easiest way I have found for inputting text like you are needing via keyboard. The open source code is here at Github in case you have any concerns. https://github.com/gonzotek/remoku
An additional help page on how to use Remoku is found here. http://help.remoku.tv/
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Feel free to post back if you have any issues or questions.
