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You can add the Action "Programmatic Buyer Selection" to your Media Campaign Page Layout
Or you can directly embed the flow into a Media Campaign Lightning Page. This way your users can access directly while editing a Media Campaign. Make sure to activate the setting to pass all fields & values into the flow variable.
Add Permissions and Modify Page Layouts: In order to use the flow, make sure that your users have sufficient permissions
For the object: Assigned Programmatic Buyers
For the object: Accounts
For the object: Media Campaign
to use Lightning Flows
You will need to adjust your Account page layouts to show the related list: Assigned Programmatic Buyers, so that you can configure a relation between Buyers and Advertiser Accounts