How to get the signature for OpenSea's "create-an-offer" endpoint?

I'm trying to utilize this OpenSea API endpoint "https://docs.opensea.io/v2.0/reference/create-an-offer"

I don't know what I've to send in signature. I tried sending "0x" but it's saying { errors: [ "['Invalid order signature']" ] }

If you know how to fix this. Do let me know!