Se kilde

To Set it:

import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.customFieldManager
def crtField = customFieldManager.getCustomFieldObjects(issue).find{it.name == "Customer Request Type"}
issue.setCustomFieldValue(crtField, "Desired Request Type")

To Clear it:

import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.customFieldManager
def crtField = customFieldManager.getCustomFieldObjects(issue).find{it.name == "Customer Request Type"}
issue.setCustomFieldValue(crtField, "Desired Request Type")