import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.security.roles.ProjectRoleManager
import static com.atlassian.jira.issue.IssueFieldConstants.ISSUE_TYPE
def issueTypeField = getFieldById(ISSUE_TYPE)
def projectRoleManager = ComponentAccessor.getComponent(ProjectRoleManager)
def availableIssueTypes = []
availableIssueTypes.addAll(["issue type1", "issue type2 etc"])
issueTypeField.setFieldOptions(availableIssueTypes)