It's not a good idea to give your desired salary AND to add "negotiable". Because that tells the company that you will be accepting less, which weakens your negotation position instantly.
If I say "$80,000" and you say "$80,000, negotiable", when we both think "$80,000 would be very nice, $75,000 would be acceptable, and $...
It is absolutely acceptable to write "negotiable" and not give them a number. Some schools of thought in fact recommend avoiding giving a number.
Some articles following this school of thought: