1. I would have gone to the party if I had known about it.
2. I would have gone to the party had I known about it.
Answer
"Had I known" works in exactly the same way as "If I had known".
'Had I known' is more formal, so you might be less familiar with it, but you can use it in Speaking and Writing.
In my YouTube video about 3rd conditional, you can skip forward to about 08:34 mins and find some IELTS related examples that use this 'Had + Past Participle' version of the Third Conditional.