With Sitecore 9 Forms, is there any way of adding a multi-select drop down list?
The Dropdown list control does not support multiple select. Even if it did, from a UX perspective, that's not the control you would want to use. A dropdown indicates to the user that you want only one option selected. For multiple select, a form should use something like a checkbox list or a list box to indicate visually to the user that multiple answers are expected.
In this case, the most likely control you want is a List Box.
Use the List box element to display a list of options where a contact can select any number of choices, including zero, one, or several.
It contains a Selection Mode property which you can use to change it to allow multiple select:
Selection Mode: Indicates whether only a single item or multiple items can be selected from the list at one time. The options are Single or Multiple.
Alternatively, if the number of options you have is small, you could use a Checkbox list control. It is always intended for multiple select, but from a UX perspective you should only use this when you have a small number of items to select from.
- Documentation landing page: https://doc.sitecore.net/sitecore_experience_platform/digital_marketing/sitecore_forms
- The list elements: https://doc.sitecore.net/sitecore_experience_platform/digital_marketing/sitecore_forms/creating_forms/the_list_elements
- Adding list items to forms: https://doc.sitecore.net/sitecore_experience_platform/digital_marketing/sitecore_forms/creating_forms/add_list_items