I think I just figured it out.
Services > DHCP > ServerSettings
Code:
{
"option-def": [
{
"space": "dhcp4",
"name": "vlan-id",
"code": 132,
"type": "uint32"
}
]
}
Then hop over to the interface, in my case: PHLAN
{
"option-data": [
{
"name": "vlan-id",
"data": "10",
"space": "dhcp4"
}
]
}
Hope this helps someone! I don't have enough permissions apparently to delete my own post LOL.