How can i allow my visitors to upload file?

I want my visitors to upload files. To be more specific JPG file. I want it on my pop up sign up form. is there any code that i could use? I know nothing about coding. but i do have a code that i used on my "add to cart" page that allow customers to uplaod JPG file.

This is one solution for adding an upload field to a form in Squarespace:

https://www.squarewebsites.org/blog/squarespace-websites-uploader

@Ziggyhello, thanks for responding. I already have this code in my code injection. But its being used in checkout page. You know when you click on "add to cart". I have been pulling my hair for past 1 hour and still couldnt figure out what is wrong am i doing. I want in my form.