Free Trial API?

What kind of API key do I have to generate for the free trial? I tried two different one — one with a $0 limit and one with a $1 limit, but whenever I try to execute something in Blackbox it tells me I have insufficient credits: https://i.imgur.com/NNCVFZk.png

Also, the pricing is a bit confusing. It says you get $50 worth of usage for the $20 plan. Huh??? How do they calculate the $50?