How to enable VXLAN config: DirectRouting=true

Hi,
I saw flannel options there when create new k8s cluster
network:
mtu: 0
options:
flannel_backend_port: ‘4789’
flannel_backend_type: vxlan
flannel_backend_vni: ‘4096’
plugin: flannel

but I can’t find any option about enable “DirectRouting”.
May I know how to config this parameter when create new cluster ?
Thank you.