All tests send form-encoded. Trying different combinations to get past the 400.
Test 1: No TEST flag
Just credentials and name. No TEST param at all.
Test 2: TEST=0
Credentials + name + TEST=0 (live mode explicitly)
Test 3: TEST=1
Credentials + name + TEST=1
Test 4: Full details + TEST=1
Name, email, phone, address, postcode + TEST=1