how to set Etherium itself as an underlying

[vsmelov]

Suppose, I want to trade ETH against some Token. What address should I put as an underlying?

[mxn]

Maybe I misunderstood. in case of ETH one needs to use Wrapped-ETH contract. Which underlying to set: depends on what do you possess and what your strategy. Option writing limits your return in case of the rapid growth of the underlying but allows to get profit even if the underlying went down. Contrary possessing of (call) options can expose return in case of growth. You can read more detail in Wikipedia article. Of course I would be glad answer further questions