Ospf over ipsec tunnel

Hi, We have been deploying a few 60Fs at Branch location which have IPSec tunnels back to HQ Fortigate devices.

So for example, I'm looking to have tunnel from branch FGT to HQ DC 1.

I would like to use OSFP to advertise the branch LAN back into the HQ network over the IPSec tunnel.

The basic layout would be BRANCH CORE->BRANCH FGT --> HQ FGT --> Cisco Core.

So I would like to advertise the branch Lan back into the Cisco core over these VPN Tunnels. We have OSPF running on our Both Cisco Core but I have never used this on Foritgate devices.

Could someone please explain the process or point me in the right direction