Short answer: You don't. You can threaten to take it to social media on your support ticket, and maybe threaten a chargeback in the support ticket. That often gets their attention.
Small warning: avoid actually doing a chargeback at all costs. If you do it without permission from Steam, expect your account to be permanently banned, and with their already awful support you're probably never going to get it back.
Small warning: avoid actually doing a chargeback at all costs. If you do it without permission from Steam, expect your account to be permanently banned, and with their already awful support you're probably never going to get it back.