Recently I had to figure out how the enable routing between a FortiGate VM in Azure and an Meraki vMX. They were located in the same subscription. There is also a Routing table available. To other subnets and vnets in other subscriptions.
On the Meraki vMX you need to add the subnet which you want to publish trough the VPN to the other sites. Which are connected to the Meraki AutoVPN
When you have added this subnet you go to the FortiGate Firewall to add the subnets which you want to reach from the SSL VPN Client
Add the subnet to the gateway of the internal Lan side of the FortiGate
Add the subnet to the firewall policy on the FortiGate this way the FortiClient will learn the subnet.
Ensure you have added the the subnets to the Routing table and pointed to the correct virtual appliance.
Ensure you will connect the subnets of both virtual appliances to the Routing Table. if you for get this step your routing won’t work