Can a girl do mutah without informing her parents to a Muslim boy as she is 21years old

According to seyed seestani as long as the girl is still under her father’s care (not independent) she needs his permission.
And if she was independent, she still needs permission but as an obligatory precaution. That means in this case(being independent) you have the choice to follow seyed seestani in his obligatory precaution, or you can follow the most knowledgeable scholar after him in this matter.