The field phonetic spelling is optional and some people skip it during registration.
If you would like to provide more information about the pronounciation of your name, you can fill it in however you want. Think about what would be the easiest way to explain how to pronounce your name.
For example, if your surname is "Green", you can write: Green like the color.
Other examples of words and their phonetic spellings are: easy (ee-zee), alphabet (al-fuh-bet), July (joo-lahy).