The correct option is Purchase services.

The Purchase services page is where administrators add new Microsoft 365 subscriptions and adjust license quantities. This page walks administrators through buying services and links to the license management workflows so that purchases can be followed by assigning or reassigning licenses to users.

Billing is focused on invoices, payment methods, and billing profiles and it does not provide the purchase workflow for new subscriptions or the interfaces used to assign licenses.

Tenant settings are for organization level policies and configuration and they are not used to buy subscriptions or directly manage user licenses.

Products and subscriptions lists the subscriptions you already have and shows status and details but the procurement and add subscription flows are handled on the Purchase services page rather than on that listing view.

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