As with anything else related to WTBs, how you fasten the string depends on what you're using - for both the tub and the string - and on how you've set it up to play.

If the string tension will not be too high, and if the tub material is strong enough, a simple knot in the end of the string (after you slip it through a hole) works fine.

If the knot pulls back out, or if the tub material "tears" out, you can tie something on the end of the string that's large enough to resist pulling through. Running the string through a small washer inside the tub, and knotting the string so that the washer is pulled back against the tub when you apply tension, may be sufficient.

For really "high-tension" or in soft material like plastic buckets, you can use the 50-cent setup consisting of a small "eye-bolt," with a "fender washer" and nut on each side of the tub, so that the eye-bolt is clamped to the tub. The string can be tied through the eye of the bolt that sticks up on the outside of the bucket.

(The fender washer has a hole just like a normal washer but a larger outer diameter to resist pulling through thin material. Your "hardware store guy" will know.)

John