Warpcast

Trying to add my SOL wallet to Warpcast and this is the process?  Enter my private key?  Basically goes against best practices.

Why can't it prompt Phantom and just let me sign?

I don't think this is true  when I linked my Phantom wallet was back then 

I was  prompted to sign on my wallet and link the phantom wallet to my warpcast acct  

So be sure you are using the right website for this interaction to avoid being hack