Populate the customer profile attribute when you sync users from other okta account

We are building the application where the user identity will be managed in OKTA account. The users of the application will come from multiple vendors. Each vendor might have their users in OKTA account. We want to sync the users to our OKTA account using Generic OKTA to OKTA IdP. When syncing we want to populate the customer profile attribute field say “vendorName” (vendorName: vendor1). As far as I explored, once user profile is created in main okta account, we can populate the customer profile attribute via UI or via API. How can achieve this in an automated way without intervention ?