Where can I enable, "Allow Apps from Untrusted Developers", permanently in Mac OS Sierra?

There is no option to enable the box allow apps from untrusted developers permanently ( in security & privacy ) , but you can read this article : macOS Sierra: Open an app from an unidentified developer

To override your security settings and open the app anyway:

1. In the Finder, locate the app you want to open.Don’t use Launchpad to do this. Launchpad doesn’t allow you to access the shortcut menu.
2. Control-click the app icon, then choose Open from the shortcut menu.
3. Click Open.The app is saved as an exception to your security settings, and you can open it in the future by double-clicking it just as you can any registered app.

Note: You can also grant an exception for a blocked app by clicking the “Open Anyway” button in the General pane of Security & Privacy preferences. This button is available for about an hour after you try to open the app.

To open this pane, choose Apple menu > System Preferences, click Security & Privacy, then click General.

To get the 'Anywhere' option in Gatekeeper back open the terminal and type

sudo spctl --master-disable

Then enter your password when prompted

and to put Gatekeeper back to how it was then in the terminal type

sudo spctl --master-enable

There is no reason to do this. It is pretty easy to run an unsigned application the first time: control-click, choose Open and confirm. This is only necessary for downloaded applications the *first* time they are used. Not an inconvenience at all. Disabling the OS safety features for this is a bad move.

You may not do that without modifying macOS in a manner that I do not recommend.

Please read "How to open an app from a unidentified developer and exempt it from Gatekeeper" in OS X: About Gatekeeper - Apple Support.