What do you mean with 'activate'? Do you want to values to be selected when opening the document? You can do that more easily by creating a bookmark with the selections you want and activating the bookmark on opening with an Activate Bookmark action assigned to the OnOpen document trigger.
For the Select in Field action you enter the name of the field in which the selection must be made in the Field input field. In the Search String input box you can enter an expression that returns a list of values. The list of values must be enclosed in parentheses and separated by a pipe symbol: '|'. An example search string is ("A1"|"B4"|"Cx")