Setting Up Presence/BLF Buttons (AT&T Office@Hand)

In your AT&T account, where you see Admin Portal in the top right, change to My Extension:

(If you are setting up buttons for another user, remain in the Admin Portal, go to Users, find & click on the user, then you will already be in the "Settings" section - proceed with the following steps at "Phones & Numbers.")

Then go to Settings --> Phones & Numbers --> Presence button. 

Then, in the list that appears, basically each numbered row is a separate button.  Click Assign Extension, search for & select the desired user, then click Done.