Create checklist items using "getSingularObjectFromString"

The best way to create a Checklist Item object is to use the getSingularObjectFromString(String) method of the Custom Field Type. The input to the method is a string containing a Json representation of the item.

import com.atlassian.jira.component.ComponentAccessor;

// Retrieve the Custom Field Type for the Checklist Custom Field
def customFieldManager = ComponentAccessor.getCustomFieldManager();
def checklistCustomField = customFieldManager.getCustomFieldObject("customfield_10013");
def checklistCustomFieldType = checklistCustomField.getCustomFieldType();

// Json representation of an item
def itemJson =  '''
{
	"name": "Run all tests",
    "checked": false,
    "statusId": "none",
    "mandatory": false,
    "rank": 0
}
''';

// Create a checklist item
def ChecklistItem newItem = checklistCustomFieldType.getSingularObjectFromString(itemJson);

Utility Method

If you intend to create many items, a good practice is to use an utility method to facilitate creation of a ChecklistItem objects.

def ChecklistItem createChecklistItem(ChecklistCFType cfType, String name, boolean checked, String status, boolean mandatory, int rank) {
    def itemJson =  """
    {
        "name": "${name}",
        "checked": ${checked ? "true" : "false"},
        "statusId": "${status == null || status == "" ? "none" : status}",
        "mandatory": ${mandatory ? "true" : "false"},
        "rank": ${rank}
    }
    """;
    return cfType.getSingularObjectFromString(itemJson);
}