I think PD Tutorials thought you were asking about PD, not AD.
You're right that the normal editing tools don't allow you to add any new silence, but you can do exactly what you're looking for by placing the scrubber at the beginning of the clip, then going to the Adjustment panel on the Edit screen. Expand the Insert Silence tool and you should see the duration already set to 1.000 seconds.
Click Apply and you're done
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