How can I schedule WhatsApp messages without unlocking my phone?

[Android Central Question]

Most of the existing apps on the market catering to scheduling WhatsApp messages require unlocking your phone (SKEDit, Do it later, etc). I find that defeats the purpose as most people will not be available to send the message in real-time and manually - thus needing such service. Does anyone know a workaround that does not require leaving your phone unlocked for the scheduled message to go through?