Reference: https://wiki.mikrotik.com/wiki/Manual:Interface/Bridge#Bridge_VLAN_Filtering
This setup is basically a combination of Example #1 and Example #3.
Physical connections:
Port 1 - To BTU
Port 5 - to IPTV box.

# -------------- PPPoE configuration ----------------------- #
# Create  VLAN interface VLAN500 for PPPoE.
/interface vlan
add interface=bridge mtu=1472 name=vlan500 vlan-id=500


# Create PPPoE interface and attach to VLAN500.
/interface pppoe-client
add add-default-route=yes allow=pap,chap disabled=no interface=vlan500 name=\
   pppoe-out1 password=xxxxx use-peer-dns=yes user=xxxx@unifi

# -------------- VLAN configuration ----------------------- #
# Using default bridge.
# Disable VLAN filtering first.
/interface bridge set bridge vlan-filtering=no

# Configure PVID 600 on port 5. This is for traffic from IPTV box to Mikrotik.
/interface bridge port
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=wlan1
add bridge=bridge comment=defconf interface=wlan2
add bridge=bridge comment=defconf interface=ether1
add bridge=bridge interface=ether5 pvid=600


# Create 2 VLANs 500 (PPPoE)& 600 (HyppTV).
# For VLAN500, tag port 1 and bridge.
# For VLAN600, tag port 1 and untag port 5. This for traffic from Mikrotik to IPTV.
/interface bridge vlan
add bridge=bridge comment=PPPoE tagged=ether1,bridge vlan-ids=500
add bridge=bridge comment=iptv tagged=ether1 untagged=ether5 vlan-ids=600


# Important! - Enable VLAN filtering. Without this PVID will not be in effect.
/interface bridge set bridge vlan-filtering=yes


# Check system logs. Wait for PPPoE to connect.
# Reboot your IPTV box.