If it will let you type in “negotiable” go with that. If it won’t, but it will let you move on without answering, leave it blank. If you are forced to answer, you’ll have to do some market research to find out what they generally pay, and what your qualifications are compared to their typical employee, then give it a reasonable response. I never hire or refuse to interview anyone over that question, and I rarely consider their answer in determining what I feel is a fair offer of salary. It just gives the hiring managers an idea of what you are looking for (far less or far more than they will offer).