You want to use Partner Interconnect to connect your on-premises network with your VPC. You already have an Interconnect partner.
What should you first?

A. Log in to your partner's portal and request the VLAN attachment there. B. Ask your Interconnect partner to provision a physical connection to Google. C. Create a Partner Interconnect type VLAN attachment in the GCP Console and retrieve the pairing key. D. Run gcloud compute interconnect attachments partner update <attachment> / --region <region> --admin-enabled.