From here, I understand a custom api with custom action is doable on dynamics 365.
And according to here, custom auction by code is only supported by on-premises/IFD server;
But the next section says you can pack action by code in solution to distribute it as well. (which 365 online can import a solution)
This is confusing. My question is - is it doable? If yes, is there any guide on this? I have no idea on how to start coding it, the second link above look like a section of code to me and not complete.