How do you keep the bathroom sink from leaking from the bottom when hooking up a portable washer to it?

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How do you keep the bathroom sink from leaking from the bottom when hooking up a portable washer to it?
Some questions first. The bottom of what? The lavatory connection to the pop-up waste? The trap connection? And what about this?
hooking up a portable washer Do you just drain it over the lavatory rim or have you replaced the tailpiece with a branch tailpiece, (see image) and connectede to that? In short, you have gave us nothing to work with. We need more details before we can help. Cheers, Tom