Here is what you need to do on your side:

  1. Open the BM (Business Manager). Use one you already own that has a free ad account slot, or create a new BM for this client.
  2. Inside the Business Manager, create the ad account. You don't need your client's login or password for this.
  3. Create the company's Facebook Page (or connect the one it already has) and add it to the BM.
  4. Link the client's Instagram to the Page. This is the only step that depends on the client: they sign in to their own Instagram account to authorize the link (you don't need their password).
  5. Add a payment method to the ad account: a credit card, or Pix (Brazil's instant payment method) to add funds.
  6. Done: you can launch campaigns. Then just connect this account to Ads Editor to manage everything here.