The example below will allow you to add a hidden field to your form while customizing the field name and field value.

Use this code to remove all used options from a dropdown, radio, checkbox, or Dynamic field.

If you want a separate form for adding categories, you can use this code to create them. If you want to keep the category creation inside the same form that creates posts, this custom code is not needed. Use a "Tags" field instead, and map it to your category in the "create posts" settings tab.