I need to show pfm users that they can perform some mathematical operations directly in the text fields. like in Sketch App? But how to do it best. if I want to make users don't need to think about It?
Why don't you simply give them a hint:
The idea is to give a hint with a slightly faded text that disappears the moment the user starts typing their own text. Furthermore upon hesitation (user takes a break in typing) you can display a balloon (if your technology allows) hinting the syntax.
What about giving that information in the field?
I did not find proper icon in Balsamiq, something like calculator would be better fit. User gets a hint to use calculations. You can make that icon clickable to open formula builder or helper, if you are open to make one.