If I think back to in time with C/IDE we used to receive addons and custom development in the form of a FOB file. This was the norm to import them and then when you create your own stuff you'd be able to reference them as variables in your creations.
Now with Microsoft Dynamics 365 Business Central adding a variable reference to an object that resides in another extension is not as clear at first glance as there are a couple of steps you need to do to your AL solution.
To use another extension's objects and have the dropdowns working nicely you first have to reference the extension and make yours a dependant of theirs.
You can follow these steps as a guideline:
Make sure the extension you want to reference is already installed and published in your development environment. :) No open heart surgery.
You will need to collect some information regarding the extension you want to reference in the extension management window. In particular the ID, Name, Publisher and Version. Using the Help>>About This Page will show you all the fields you need here as the ID is not visible.
The information you collected can be now placed into the dependencies section within the app.json file in your AL project. It should look similar to the following:
Now comes the real trick is when you download symbols in your AL project, you'll see that you will have downloaded the extension to the .alpackages folder in your solution. From this point on you'll be able to reference the extension objects in your project.