Don't show the mobile keyboard

I’m making an app where the user needs to enter special characters and the keyboard that pops up on the iOS and Android is too tedious to use for these characters.
To solve this, I made my own keyboard using UnityGUI. My problem is that the system keyboard pops up over the one I made whenever the user focuses on a text field. Is then any way to stop the mobile keyboard from popping up when focusing on a text field? Any help is appreciated :).

Since android does not support the TouchScreenKeyboard.hideInput method from the reused IOS Keyboard API, I would suggest using a simple button or other activate-able object the user can click. This can then trigger your custom keyboard.

As they type, the text value will be updated, like a text field. This will get around having to worry about activating any text input on a device regardless of OS.