Discord API is quite useful. But their bot system is confusing. It can ONLY send messages via the API provided , the bot is connected as a regular user at least once. There is not much documentation about it.
However, there is a way out. Download a neat utility called LiveBot. It is an Electron based Windows Executable (Its over 300Mb, unfortunately – all electron programs are large) https://github.com/SebOuellette/LiveBot/tree/executable
Then run that program. Give the token for the bot you created in your account. The token is available from https://discord.com/developers/applications (again a confusing point. One would have thought that it would be available from the discordapp where you login into Discord. Not so. You need to visit that developers page)
Anyway, once LiveBot is running, you can simply paste the token there – and it behaves as if the Bot has logged in like a regular user. Once that is done, then the Bot can send out messages via API (Need to check if LiveBot needs to be run regularly, not sure about that)