The built-in Bengali keyboard layout in Windows 8/ Windows RT is not user-friendly at all. So using an IME is the only option for most users. There are quite a few Bengali Input Method Editors in Win32. They work fine in Windows 8.
But for Windows RT there are no solutions at the moment. Is it possible to write IME's using purely WinRT APIs?
A good information of writing IME is given in this MSDN Link
I putting some traits from the article which is important
Requirements for Windows 8 IMEs
A third-party IME must meet these requirements: •Must be digitally signed. •Must be Text Services Framework (TSF) aware, and proper IME flags must be set to run properly in Windows 8. •Must follow UX guidelines for Windows Store apps and be compatible with Windows Storeapps.A third-party IME that doesn't meet these requirements is blocked from running in the new Windows 8 UI, but it can still run on the desktop.
Also, Windows Defender removes malicious IMEs from the system. Because of this, it's important that you familiarize yourself with the IME coding requirements for Windows 8. For more info, see Guidelines and checklist for IME development.
The next LINK shows the Guidelines for developing IME
As you can make an app for Windows Store you would be able to access the IME app in WinRT via Windows Store.Check if any Bengali IME app is available in windows store