I don't typically use the ASDM, but I do know that you would go to Configuration -> Device Setup ->Interfaces -> Add interface.
Also, whatever device this trunk would go to would need to be enabled for 802.1q trunking as well.
Here are some basic instructions for using the CLI:
To add a subinterface and assign a VLAN to it, perform the following steps:
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Step 1 To specify the new subinterface, enter the following command:
hostname(config)# interface physical_interface.subinterface
The subinterface ID is an integer between 1 and 4294967293.
For example, enter the following command:
hostname(config)# interface gigabitethernet0/1.100
Step 2 To specify the VLAN for the subinterface, enter the following command:
hostname(config-subif)# vlan vlan_id
The vlan_id is an integer between 1 and 4094. Some VLAN IDs might be reserved on connected switches, so check the switch documentation for more information.
You can only assign a single VLAN to a subinterface, and not to the physical interface. Each subinterface must have a VLAN ID before it can pass traffic. To change a VLAN ID, you do not need to remove the old VLAN ID with the no option; you can enter the vlan command with a different VLAN ID, and the security appliance changes the old ID.
Step 3 To enable the subinterface, enter the following command:
hostname(config-subif)# no shutdown
To disable the interface, enter the shutdown command.
If you use subinterfaces, you typically do not also want the physical interface to pass traffic, because the physical interface passes untagged packets. Because the physical interface must be enabled for the subinterface to pass traffic, ensure that the physical interface does not pass traffic by leaving out the nameif command. If you want to let the physical interface pass untagged packets, you can configure the nameif command as usual.
HTH,
Paul
