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:

This works better than using JSU to "Clear Field Value"; as JSU does not 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, null)

After the script is executed, You can use "'Customer Request Type IS EMPTY" on the field.