Set a Default Option on a Select List

import com.atlassian.jira.component.ComponentAccessor
import com.onresolve.jira.groovy.user.FieldBehaviours
import groovy.transform.BaseScript

@BaseScript FieldBehaviours fieldBehaviours

// a single select list custom field name
final String fieldName = "Select List"

// the value to set
final String setValue = "Some Value"

def field = getFieldByName(fieldName)

def optionsManager = ComponentAccessor.optionsManager
def customField = ComponentAccessor.customFieldManager.getCustomFieldObjects(issueContext.projectId, issueContext.issueTypeId).find {
    it.name == fieldName
}

assert customField : "Could not find custom field with name $fieldName"

def fieldConfig = customField.getRelevantConfig(issueContext)
def options = optionsManager.getOptions(fieldConfig)
def option = options.find { it.value == setValue }

assert option : "Could not find option with value $setValue"

field.setFormValue(option.optionId)