External link is for URL navigation if you would like to make e rest api call you can use custom event actions.

Yes, but we would like to have this function as button.

is it possible to trigger event through custom button?

What you are trying to do can not be fully achieved with just configuration, you have to also have some code development. 

It has a little complicated path. First of all you have to have a API procedure which is calling rest sender. and then you can customize your projection and add this method as action. and finally you can assign that action to your custom button.

But with some minor requirement changes you can achieve the functionality like wise you can add a custom checkbox to your entity and listening on change of that field to trigger rest call. So when you mark it checked and saved the form, event will be triggered and rest API call will be placed and then programmatically update field back to unchecked state.

Hope that help, please inform us about the progress...