First I create the formatted html:
onClick={(_) => {
const content = `Nézd mit találtam ..<div></div`;
then assign content
window.location.href = `mailto:user@example.com?subject=Jelentkezz vagy oszd meg és nyerj 250$-t&body=${content}`
but in Apple Mail it looks like this:
Possible duplicate of this question.
As mentioned in the answers to this question, it's not possible to format the body using HTML in mailto
.
But there are a few other possibilities to achieve the same result. I won't repeat them here, as they are already explained in detail in the other question.