Vulnerability in EPiServer.Forms
Could you please suggest to me to implement a custom episerver form element that allows multiple selection. like a Multi-select dropdown with validation.
Start by creating a custom form block and work it out from there
You can find information about creating a custom form element block here:
As you're looking for multi-select capabilities, it might be worth having your element inherit from ChoiceElementBlock rather than ValidatableElementBlockBase.