Welcome back:
Okay, that round plug is an inspection sight for the flywheel and or torque converter. That is the bottom part of what is called the bell housing and is part of and how the transmission bolts to the engine. Behind the plug is the transmission oil pan.
If coolant is leaking from near the top of the engine, chances are it is making its way to that point. There could be an expansion plug (aka a freeze plug) on the back of the engine block between the engine and transmission that is leaking into the bell housing, but it's unlikely.
What I suggest is to clean everything off and then start filling it slowly while you watch where it comes from.
Let me know what you find, if you have questions, or just need help.
Take care,
Joe
Tuesday, July 30th, 2019 AT 6:02 PM