JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
The thing you want can be easily done on the "Form Builder". To make some of the choices preselected, please follow the steps from the below:
After those steps, you can see that related fields are preselected on the preview. Please, see it from the below:
In addition, because of the enabled "Read-Only" option, those options are not changeable as you have requested. To see section to set that option enable/disable, please check the following screenshot:
For further assistance, please do let us know.