I created a multiple select drop-down using a custom HTML. How can I add field rules to that drop down?
Is there any other way to add a multiple select drop down?
You could place the Drop down in a collection.
Please note that you cannot place a Collection inside a collection!
Technically, the only multi-value field you can place on a form is a checkbox or a single value field in a collection or table. If you wanted to use a checkbox as your multi-select, that should work. You could also use some JavaScript to hook up your multi-select drop down in the custom HTML field to a hidden checkbox. This would allow the form filler to interact with the dropdown, but it would fill the checkbox behind the scenes so Forms knows about the variable data on submission. If you connected this custom HTML dropdown field to the checkbox, you could build field rules based on the hidden checkbox.