In three easy steps.
Step 1. Say, “I went poopoo.”
Fair enough. Dad will go check it out.
Step 2. Say, “I flush it self.”
Excellent. Good communication. This way Dad isn't at a loss to unflush the toilet to stop the screaming.
Step 3. Raise and lower the toilet lid, saying (in a cookie-monster-ish voice) “I like eat poopoo. MMmmmMM.”