Once all the channels are added in your media brand, you can add offers related to it.
Go to Offers at the top of the page and click on add an offer. Select the media brand for which you want to add your offer.
Depending on the publication format chosen: specify all the attributes of the publication.
The attributes will not be the same if the publication is an article on a website or a story on Facebook.
Then you can add "relays" and KPIs (but it is not mandatory): same here, the available relays depend on the main publication and the KPI will not be the same either.