The site is 100% legitimate—created by President Zelenskyy himself. You can even choose where your donation goes.
The link to donate is below. Give what you can.
To the people of Ukraine: 6/8

UNITED24 - The initiative of the President of Ukraine
UNITED24 was launched by the President of Ukraine Volodymyr Zelenskyy as the main venue for collecting charitable donations in support of Ukraine. Funds will be transferred to the official accounts of...