17.2Start by clarifying the contract

Ask or state assumptions:

POST /customers

{
  "customerId": "C001",
  "companyName": "Acme",
  "phone": "3055551234",
  "billingState": "FL"
}

First important question:

“Do we have a stable source-system customer ID?”

If yes, excellent.

That's our Salesforce External ID.