I found these recommendations from PG&E for temporary service poles: 20 ft above ground, 4 feet below ground. This means whatever the exposed length of the pole will be above ground, you need to have 1/5 of that length below ground. You need to dig a narrow hole with a cone shaped bottom to keep the pole centered while you are setting it. Then after backfilling around the pole, ram the soil good and hard to pack it down. Maybe do that several times as you fill in the hole to make sure all the soil around the post is fully packed. Whatever length pole you have, you probably should have at least 3 feet of the length set underground, but for poles longer than 18 feet follow the 1/5 rule.
Guying wires might be necessary until the ground has fully hardened.
https://www.pge.com/includes/docs/pdfs/ ... 025055.pdf