Can I add an action without any plugin?

I think you would have to be a little more specific. Not that I know the answer, but I think you have made it difficult for anyone to answer because the question is too broad.

My function is to change the text into a special characters if the original text is followed by a specific number, so that people don’t have to install a special input method, and can still post by adding a number to English text when they mention some special characters.

I want to achieve this: Clicking Submit button will call myfuntion()

How to do this?

Thanks for helping!