I was gonna say the heater hose connections at the firewall but if you are adding fluid and the leak starts as soon as you start pouring in the fluid, then
@exp500 is likely right. Remove the inspection cover at the front of trans and watch again while pouring fluid in. There are freeze/welch plugs all over the engine, front, back and sides. Check them all because if one is leaking then others may be ready to leak through where the rust/corrosion is at. Would not hurt to look up at the back of intake on each side or from the top with a mirror if needed. Also the head gaskets. May need to do a compression test for that.