Desktop/homepage shortcut to send pre-built email in one click
Automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros and you need only one macro for your need) 
and easy to learn.


Give the app all permissions it asks for and from Settings &rarr;Email settings, enter your Gmail account details.
Create a macro as shown below (use + to add triggers /actions). In the send mail action fill in details of recipient and the text of mail. In pop up action enter text that you want to be displayed when you create a widget (explained in next step). Save the macro with any name. 




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From the widgets drawer of your launcher drag and drop MacroDroid Shortcut widget on top your screen. On the menu that opens up, assign it to the macro you saved.


Tap the shortcut to send the mail. I've tested and it works fine.



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You may edit the icon of widget to a custom one and add more bells and whistles by way of adding a sound action, scheduling to send mail at fixed times /days, or whatever you want ( endless possibilities) 
later