First, download the site and transfer it to an offline computer, preferably one which will never go online again and is running a live OS such as Tails. You don't want to take the risk of your seed phrase leaking to the internet and losing all your coins.

Enter your seed phrase in the top box titled "BIP39 Mnemonic". Then enter your passphrase in the next box titled "BIP39 Passphrase (optional)". Scroll down to where it says "Derivation Path" in big letters, and select "BIP44" if the hidden address begins with "1", select "BIP49" if it begins with "3", or select "BIP84" if it begins with "bc1". Then scroll down again and see if the address is there. If not, try a different passphrase.

If you still cannot find the address despite trying every passphrase you can think of, then you might want to consider using a program such as btcrecover to attempt to brute force it.