Custom Multiselect fields can be added by clicking on the Plus button in the Field manager, on both Organization and Event levels, and selecting the option Multiselect in the dropdown offered.
Planners can edit the name of the field (e.i. Preferred activities), label for the field visible to contacts and participants (which may or may not be the same as name of the field) and the category the field belongs to (e.i. Participant agenda).
In order for this field to work, planners need to add possible values in the Validation section that will represent options offered for this field (e.i. Sales Workshop, Product 101, Keynote speeches etc). One or many options will be available for selection.