mrb.el

(register-input-method
   "MyTeX" "UTF-8" 'quail-use-package
   "\\" "Personalized TeX input method"
   "~/.emacs.d/mytex-inputmethod")
   
  (defun mrb/set-input-method()
    (interactive)
    (setq default-input-method "MyTeX")
    ;; Toggle only if it not active yet
    (if (not current-input-method)
        (toggle-input-method)))

  (add-hook 'set-language-environment-hook 'mrb/set-input-method)
  ;; And now we can set it
  (set-language-environment "UTF-8")

  (add-hook 'text-mode-hook 'mrb/set-input-method)