How can I get a private key for this address
There is essentially no way.
Is it possible for me to withdraw my bitcoin I invested here? I don't have the private key
No.
It looks like you fell for a common scam.
- https://bitcoin.stackexchange.com/q/73817/13866
- BTC Stolen from Blockchain
- "Hidden Wallet" onion site
- I've been asked to share wallet credentials to participate in mining. Is that a scam?
- Recovering the private key from a blockchain wallet address
- Where do I obtain private keys?
About watch-only addresses