Show instruction on field base on dropdown field

Hi

looking for an ideas how to accomplish this one.

I have search form with.
dropdown (list) and
textfield

I want to show some instructions like what to type on textfield based in my dropdown field. I have an idea like placeholder in textfield. but it doesn't change everytime.

is it possible that the placeholder will change everytime i choose in selection? or if not a container maybe.

thanks