Is there a way to call AJAX from a widget on the server side instead of client side?

Are you talking about calling an external api via custom serverside code?

What's the use case for needing to call it on widget load?

We have a custom API we created that we need to call when the widget loads, but we don't necessarily want to expose it to the outside world.

If you create a Widget Extension to call your API from the widget, it can resolve at render time and not be exposed as is. You can also use ExecuteFile to perform a postback instead of using a REST endpoint.

If you create a Widget Extension to call your API from the widget, it can resolve at render time and not be exposed as is. You can also use ExecuteFile to perform a postback instead of using a REST endpoint.

Perfect. This will work just fine