https://answers.atlassian.com/questions/190610/how-to-create-post-workflow-function-to-update-field-of-linked-issue

gistfile1.groovy

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue
import org.apache.log4j.Logger

def log = Logger.getLogger("com.onresolve.jira.groovy.MyScript")

def MutableIssue issue = issue
log.debug(issue)


def fieldName = "My Multi Checkboxes"   // Change name of custom field
def optionValue = "Maybe"               // Name of option value

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def optionsManager = ComponentAccessor.getOptionsManager()

if (issue.priorityObject.name == "Blocker") {
    def cf = customFieldManager.getCustomFieldObjects(issue).find {it.name == fieldName}

    if (cf) {
        def fc = cf.getRelevantConfig(issue)
        def option = optionsManager.getOptions(fc).getRootOptions().find {it.value == optionValue}
        if (option) {
            issue.setCustomFieldValue(cf, [option])
        }
        else {
            log.warn("Could not find option with value \"$optionValue\"")
        }
    }
    else {
        log.warn ("CF $fieldName was not associated with this issue")
    }
}