import com.atlassian.jira.component.ComponentAccessor
// Name of the custom field to change
final customFieldName = "TextFieldA"
// New value for that field
final newValue = "I love Groovy !"
def customFieldManager = ComponentAccessor.customFieldManager
def customField = customFieldManager.getCustomFieldObjects(issue).find { it.name == customFieldName }
assert customField: "Could not find custom field with name $customFieldName"
issue.setCustomFieldValue(customField, newValue)