Hi,
See below for command I'm interested in.

/interface bridge
add admin-mac=AA:BB:CC:DD:EE:FF auto-mac=no name=mybridge

Assuming AA:BB:CC:DD:EE:FF is a MAC address of the ether1 and I'm planning to make ether1 as a master of mybridge.
I want to covert above command to look something like,

/interface bridge
add admin-mac=[ /interface ethernet print mac-address where name=ether1 ] auto-mac=no name=mybridge

Now I know above command does not work. But it should be very clear what I'm after here.
Is there a way to do this so I can have it permanently written inside my default config file and use it across many devices?

You was nearly there with your code.

Excellent. !!!

While we are on the topic, I woud like to know if I was to use auto-mac property would it effectively do the same thing?
Meaning, can I drop the admin-mac entirely and rely on ROS to do the right thing?

/interface bridge
add auto-mac=yes name=mybridge

I'm guessing that ROS will automagically pick up the MAC address of the very first interface that joins the bridge right? Or, is this not a guaranteed behaviour?

Using auto-mac, the MAC can change after a reboot. Not so great if you are using the MAC for static lease or for other hard coded settings.

Using auto-mac the bridge MAC address can also change if you remove the "donnor" port from the bridge. Having bridge MAC address set statically could show even nastier side-effects in such case.
The best approach would be to set bridge MAC address to a completely different MAC address for which you're 105% sure it'll never show up in your network. Such as MAC address of a physically broken ethernet NIC that happens to lie in the back of your bottom drawer instead of being disposed off properly years ago.