Here’s Tim’s tweet:

Prayers for Texas and all those affected by #HurricaneHarvey. Join us in the relief effort by donating: https://t.co/QLBMFOjaDG

— Tim Cook (@tim_cook) August 27, 2017

The link embedded in the tweet takes you to iTunes so you can make a donation to the American Red Cross specifically targeted for Hurricane Harvey relief. I believe this particular path only works in the US.

From the iTunes page:

Click Donate below the amount you wish to contribute and iTunes will transfer 100% of it to the American Red Cross, which is providing relief efforts for Hurricane Harvey.

100% of the money goes to the American Red Cross. Apple doesn’t take a penny.