Taken from K2 content Module:
<fieldset name="basic" addfieldpath="/administrator/components/com_k2/elements/">
...
<field name="category_id" type="categoriesmultiple" default="" label="K2_SELECT_ONE_OR_MORE_CATEGORIES" description="K2_SELECT_ONE_ORE_MORE_CATEGORIES_FOR_WHICH_YOU_WANT_TO_FILTER_AN_ITEMS_LIST_SELECT_NONE_TO_FETCH_ITEMS_FROM_ALL_CATEGORIES"/>
....
</fieldset>