If I want to leave for a few days, how can I ask my boss for leave?

Tell the boss directly, but it also depends on the company's system! But don't call in sick, or it will be very troublesome!

I generally agree that employees should take time off according to their actual needs. Even on a business trip, as long as your work can be arranged or the company can make a good handover, the boss will usually allow you to go out for a few days!