In order enable the multiSelect, you have to pass the correct value to the lookup config.
Here is an example of code that executes before the exact lookup was opened:
In case you change multiSelect value to True you'll get the option to choose many values in the same lookup field.
Dear Lisa, could you please send more details or an example of the MultiSelect Lookup option? thanks!
Please see the "Recipient" field in the EmailPageV2. It is a multi-lookup field, which populates values from Contact object. However, please keep in mind, that you will need to display values to a text type field, but not a lookup type. Thus, consider creating an extra text field to store selection to.