The propaganda technique is slogan because it keeps the words associated with a brand.

The speaker isn't anyone, but the message of the word from people who say it.

Slogan

The target audience is people who aren't in the best shape and are having trouble getting exercise done.

The overall purpose of this ad is to associate the taking of action with the Nike brand.

The slogan persuasion technique helps sell the overall Nike brand because the simple phrase "just do it" will make the person think of them.