Is it possible to find what is the random generated key if I know the plaintext and ciphertext? Is it possible to find what is the key with only plaintext and ciphertext?

For example,

My plaintext is: hi

My ciphertext is: 0x1db5
If you don't specify the stream cipher in question, then no.

If you do specify the stream cipher in question, and it meets some of the most modest security goals of a stream cipher, then no.