How do I access (refer to) the private SSL key for decryption?

I am integrating my wix website with 3rd party APIs for business purposes. Data is shared with this 3rd party via their API, and a response is received. This exchange of data is encrypted and decrypted using SSL certificates.

The 3rd party sends the response data by encrypting it using my wix website’s SSL public key. I need the corresponding Private key to decrypt the response data?

How do I access (refer to) the private SSL key for decryption?