You don't need to and shouldn't ask for the client's Instagram password. Ask them to sign in to their own account to link their Instagram to their Facebook Page or grant the necessary access through Meta's business portfolio.

The ad account, Page, and payment method must be in the correct business portfolio, with each person using their own login and appropriate permissions. If Instagram is not already linked, you can set up the structure and advertise only on Facebook; include Instagram after the client securely completes the link.