While this will definitely vary by review board, my personal experience has been that it's a standard that you are explicitly required to inform participants at an appropriate point (disclosures, usually) that they are allowed to refuse to answer any question they want by leaving it blank or marking it in some fashion. Sometimes this even means that, if they leave any question off, you have to throw out their whole set of responses because it's useless to you, but they must still be paid/credited for their participation even if it makes it useless to you.
The question is both ethical and practical - if you require an answer you may also be encouraging dishonesty, and possibly invalidating the usefulness of your survey anyway. "I'm not answering that" is better than "I'm not comfortable answering that truthfully, so I'll just put down nonsense." And this set of rules actually goes far beyond just the idea of unpaid volunteers, as it applies to things like medical patients, paid employees, etc.
Now, this is not to say you can't do it - ethics review boards have approved electrocuting and burning people based upon appropriate balancing factors, so it's up to the board and how you design and present your study.
But setting form fields as "required" will require appropriate disclosure to the participants and review board, and will likely be considered a negative that must be appropriately justified and handled. You will have to show that autonomy, right to withdraw, informed consent, non-coercion, and ethical payment are all handled appropriately. You will be making things hard on yourself, so if you don't have to do it this way, you probably shouldn't.
You are generally welcomed to pro-rate payment, so if a person refuses to answer all items you only have to pay them for the items they did answer - and you must put this in the informed consent forms and present this to the review board. Whether or not the particular online payment system or setup you use will support such a method might restrict your realistic study design, but that hardship is probably not something the review board will be particularly concerned about.