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To my understanding you are now trying to use the custom injected css code on "another form" which is "different" from the previous and you're unable to see it's input number.
You are correct in thinking that they are not always the same for all forms. Usually on cloned forms they stay the same but not always such as if the form was modified.
For your case though, you can actually obtain that particular number quickly by inspecting your form's source code by right-clicking in the empty white space somewhere while in the formbuilder (editor). See my Screenshots below for example. You'll then want to look through for your donation input field.
I also forgot to mention you can additionally refer to this guide here http://www.jotform.com/help/146-How-to-find-Field-IDs-Names for additional methods through the form properties but depending on your css it is better to check in the source code or more directly through our form designer's CSS Helper.