boot – Make existing external ubuntu drive bootable on mac os

I have one Windows PC and Macbook.
I have installed Ubuntu on an external drive by using my Windows PC.
I can start Ubuntu on my PC when I plug the external SSD into my PC.
However, when I plug it into my Macbook 2019 and holding CMD R, I don’t see that drive in the start up disks.

I have disabled secure boot & able to boot another SSD with Windows.
I want to do the same thing to my Ubuntu SSD.

I think the Ubuntu SSD does not have efi boot partition in it, therefore, my Macbook cannot see it as a bootable drive.

My question is:

• How can I make my Ubuntu SSD bootable in Macbook 2019 (Ubuntu already installed in the external drive)