How do I get to approve a regulation for a giveaway, from Steamgifts?

Type "your rules" in the description and make the giveaway.

Or if you're really not sure whether they're permitted just send a support ticket from the Support tab specifying what you want to do.

Make it private.

Only private giveaways are allowed to have user-specified rules

All the above answers are false or incomplete.

1) public giveaways can't have special rules.
2) all special rules require approval by a mod

You could have found this answers in the FAQ

Create a support ticket detailing your rules. If approved, create a private or group giveaway with your rules.