Jira Software (7.7 - 8.6)
import com.onresolve.jira.groovy.user.FieldBehaviours
import groovy.transform.BaseScript
import static com.atlassian.jira.issue.IssueFieldConstants.*
@BaseScript FieldBehaviours fieldBehaviours
final sprintFieldName = 'Sprint'
def sprintField = getFieldByName(sprintFieldName)
def assigneeField = getFieldById(ASSIGNEE)
assigneeField.setRequired(sprintField.value as boolean)