How to make non-disappearing placeholder (postfix in input)?

Question

css

Hello!
You need to make a postfix in the input, that is, so that after the value of the input field there is always, for example, "rub."
How can this be implemented?

Answer 1

nothing comes to my mind, how to
hang a content check on js on onkeyup or onkeydown (or onchange) and if there is no necessary ending, add it and delete all characters other than numbers and the separator of digits.
I'm not sure that the user will be comfortable with such manipulations if hung up on onkeyup or onkeydown.

Answer 2

attach the size adjustment to the input value (how to do it well is another question), remove the frame from it and add text after it. add a frame around the entire structure.
https://codepen.io/bsl-zcs/pen/QmJWXJ