In pictures :)
Add custom field:

Go to routers config:

Edit rules:

Packt Mark in:
Add key - packet-mark - value - "vlan-id-{adv_1}"
{adv_1} - advanced field ID -1

Disable rules for out:
Pmark OUT add
disabled = true

Go to customer:
Set add field to value - 3

Go to MikroTik:
1) rules for out is disabled :)

2) we see out customer field packet-mark: vlan-id-3

Now only on BETA 1.30...

Version 1.30 will be in this week.