I Have Bought A Yamahadt125 2 Stroke Does The 2 Stroke Oil Go In Petrol Tank

If it has Oil injection/An Oil resovoir, you don't need to Mix. Some even remove the Oil injection, and premix themselves. Also reduces weight. I remember the YZ's, was the DT an Enduro? Oil tank may be under side cover.
I get a Service Manual for everything I have. Can save you Time, Money, and your bike.