What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You will not be able to do this with a plain DropDown field at least not without utilizing conditions and an extra field that would allow you to do this.
On the other hand, you would be able to do this withing the dropdown and with style if you use our widget: User Contributed Dropdown
This will allow you to add your options, but also to have people filling out the jotform add the option for themselves as well.
Do let us now if that would work for you.