Same VLan on several FortiSwitches

Hi, In case of more than on FortiSwitch connected to a FortiGate, how can I put ports from different Switches into one Vlan.

As Vlans depend on Interfaces and every Switch has different Interfaces, how is this to configure? When I try to put one port of one Switch to a Vlan, only Vlans assigned to this Switch are available. Best regards