You don't tag VLAN 1. At best, I would consider the behavior there to vary across vendors. VLAN 1 is the default, untagged VLAN.
It should be untagged on mvneta0.
In Interfaces > Assignments You assign the interface you want to see that traffic to mvneta0.
When you create VLAN 999 on mvneta0 that will be mvneta0.999. That indicates the traffic will be tagged to, and must be tagged from, the embedded switch.
You would assign whatever pfSense interface you intend to be on VLAN 999 to VLAN 999 on mvneta0.
On the switch you would have:
VLAN 1, ports 1,2,5
VLAN 999, ports 3,4,5t
PORT 1,2,5 PVID 1
Port 3,4 PVID 999
In that case there will be NO tagged traffic outside the switch so any connecting switch ports must be UNTAGGED.
If you want to make, say, port 4 a "Trunk" port carrying both VLANs you would:
VLAN 1, ports 1,2,4,5
VLAN 999, ports 3,4t,5t
PORT 1,2,4,5 PVID 1
Port 3 PVID 999
The connecting switch port would need to be configured to have VLAN 1 and the untagged, native VLAN and VLAN 999 tagged.