How to add datasource for a multilist with search field in Sitecore

 The normal way of adding datasource for multilist field will not working for multilist with search. 

For multilist with search, internally the class is using a bucketted list class to search for the items to be displayed on the left pane. 

Solution:

use this string, StartSearchLocation before the datasource ID and we get the datasource searched. 

StartSearchLocation={AAAF5DC8-662A-4B8C-9E66-52EF5EEDCA20}

Also, we should note here that the data is coming from solr, so if you think the multilist field is still not displaying the items in the left pane, consider doing a rebuild of index.