Create a payment session (merchant-facing). Called by plugins (WooCommerce, Medusa, etc.) to start a BKey-approved payment flow. The merchant authenticates with a mk_live_* / mk_test_* API key. Returns an approvalUrl to redirect the shopper to.
Merchant API key issued during merchant onboarding. Distinct from OAuth JWT.
1 <= x <= 500000011360 <= x <= 7200