Congratulations! You are the first person to remove the transmission pan.
The plug is used to fill the filler tube hole in the transmission case before the unit is installed in the vehicle on the assembly line. When the filler tube is inserted, the plug falls into the pan where it remains until the pan is removed. It served its purpose and serves no useful function any more.
Many people have tried to find a place to install it or though it might be the cause of some abnormality, but alas, it’s has no current function. You can toss it.