How to customize the hint text of textboxes?

I studied tutorials about customizing the placeholder properties. I searched below code in the CSS Panel. 

::-webkit-input-placeholder {

font-size: 20px !important;
}

But there is no such a code in the css panel. What to do ?

If you cant get the code(mean probably it was added and removed or was never added) you may need to add it. To inject the css to you form kindly follow the link below:

https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes

Hope the information helps. Kindly feel free to let us know in case you need further assistance.

I need the exact code. Above codes didn't work.

It seems you want to customize the font size of hint text of your textboxes. Is that correct?

Please inject the following custom css code in your form to change the font size of textbox hint text:

.form-textbox::-webkit-input-placeholder {

    font-size: 20px !important;

}

.form-textbox::-moz-placeholder {

    font-size: 20px !important;

}

The following guide should help you how to inject custom css code in form:  https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

Yeah it is working. Thank You :-)