Scriptrunner - Add multiple issues to a Multiple Issue Picker field using groovy

[Philip Whittaker]

I am trying to programmatically update a ScriptRunner Multiple Issue Picker field using Groovy:

Here is my code:

def customFieldManager = ComponentAccessor.getComponent(CustomFieldManager);
def user = ComponentAccessor.getJiraAuthenticationContext().loggedInUser
def customField = customFieldManager.getCustomFieldObjectByName("Field Name")

Issue addOne = ComponentAccessor.issueManager.getIssueByCurrentKey("ID-123")
Issue addTwo = ComponentAccessor.issueManager.getIssueByCurrentKey("ID-321")

Collection<Issue> lists = [addOne, addTwo]
MutableIssue myIssue = issue
issue.setCustomFieldValue(customField,lists)

When I pass in a String, it adds the single issue to the Multiple Issue Picker field but I require to add multiple issues in one go. I have also tried using IssueInputParameters to update the field but have had no luck

Any ideas what I'm doing wrong?