How to know the base asset price?

How can I find the base asset price for the asset I have so I can set my stop-loss price properly?

All my orders will be made using the Market order by the way.

The base asset price is what’s specified by the quote asset.
Ex: BNBUSDT, last price is 403 USDT.
You can have a better understanding on this by reading Beginner's Guide to QuoteOrderQty Market Orders.

But what I’m actually trying to do is to sell ALL the asset amount I have whenever it reaches the stop-loss price I’ll set. How can I do this?

… Because, as far I can understand, I must provide a specific amount of USDT that I need to receive/sell. But in my case, I have to sell all the asset amount when it reaches the stop loss.

I see, there’s no equivalent of 100% quantify in the API at the moment, but the team is aware of this feedback.