DHCP Discover on VLAN100

Hello everyone - Happy New Year !!! I'm new to Fortinet/Fortigate and I might be doing something wrong - so I’m calling for your help and advices.   I'm trying to replace my Internet FAI BBox by my Fortigate 60F. The Fortigate will be plugged in ethernet directly to the FAI Fiber GPON. This is feasible with some Network and DHCP tweaks and used to worked with an OpenWRT. So my issue here is more about how to set these options up on my Fortigate rather than improving the tweaks.   Net and DHCP required tweaks are : Set 801.q VLAN 100 ; the interface mac address set to the one from the FAI BBox (MAC Spoofing) ; DHCP client must use Vendor Class identifier (option 60) set to BYGTELIAD ; the DHCP Client identifier (option 61) set to the BBox mac ;   Here are my settings (I simplified the code here, removing the next and end...)   I have an interface wan2 plugged to my FAI GPON:

set macaddr xx:xx:xx:xx:xx:xx

(whatever my BBox MAC is) Then I add a VLAN Interface "InetVlan100" with the following options

set mode dhcp</p>
<p>set dhcp-client-identifier "xx:xx:xx:xx:xx:xx"
config client-options
  edit 1
    set code 60
    set type string
    set value "BYGTELIAD"</p>
<p>set interface "wan2"</p>
<p>set vlanid 100

After some failed tries, I even tried to add

config client-options</p>
<p>  edit 2
    set code 61
    set type hex
    set value 1xxxxxxxxxxxx (whatever my MAC is, with the prefix x01 for ethernet)

And this doesn't work   When capturing the packets (using the GUI) I see the DHCP discover packets going out. However they are NOT taggued 801.q with the VLAN id 100 (as seen in Wireshark) How should I make sure that all packets going out that interface are taggued VLAN100 dot1q ? Am I doing something wrong in the VLAN definition ?