When building the engine from source, make sure that ENABLE_GFX_IME definition is set in ConfigScaleform.hThis will link in the Scaleform IME support library.
Scaleform IME support library is only available for Windows.
For EaaS users, the engine is shipped as pre-compiled binaries. These binaries have Scaleform IME support.
At this point, IME support is available in the binaries, but it's not necessarily active.
You can edit game.cfg for your project and set the CVar: sys_ime=1
Note: IME support is never available in the Sandbox, it can only be used in the game launcher.
Related information: IME