I wish i knew the answer

I'm assuming you mean in Processing JS

The basic idea is to draw a box and then draw text on top of that box, with the text being a string that gets added to whenever the user types, the optional 4 and 5 parameters to the text command make it somewhat simple to restrict long text from going outside the box.

Here's one example I had in my browser history https://www.khanacademy.org/computer-programming/textbox/5947532824281088

Or here's one I did myself (along with a bunch of other things for demo purposes)

https://www.khanacademy.org/computer-programming/processing-js-examples/6464803188899840

They aren't all that advanced implementations, but they'll probably get the job done if you just need 1 or 2.....

you can do it in html like this 

<input type="text" id="what ever you want" name="subject" placeholder=" subject"

id tag is optional i hope this helped!