The best way is to give your waistband little chance to show at all.  That depends on what you're wearing, but bikinis and other lower-rise styles will work better than full briefs or thongs.

Of course you can always use tucked-in shirts, other layers, and belts to keep your underpants under your pants.

For plausibility as men's underwear, relatively plain solid colors are always good -- white, black, maybe navy or a couple others -- in cotton or similar fabrics.  Satin in pastel pink, maybe not.

I haven't worn these myself, but it's a good example of what I mean: https://www.soma.com/store/product/cotton+modal+tailored+bikini/570345168?color=001&catId=cat40106