You need to generate an .OSG file from whatever 3D Modelling/Animation tool you are using. You can use that .OSG file together with ARToolKit5.
For export from Blender have a look here: https://github.com/cedricpinson/osgexport
Then you need to read through this C-Interface that ARToolKit5 uses to interact with OpenSceneGraph(OSG) http://artoolkit.github.io/artoolkit5/doc/apiref/arosg_h/index.html
There are several arOSGSetModelAnimation* functions that you can use to manipulate the animation.
Have a look at the Android example: https://github.com/artoolkit/artoolkit5/tree/master/AndroidStudioProjects/ARNativeOSGProj
The C-Classes show you how you can load and use .OSG files with ARToolKit.
However, you need to use C on iOS and you cannot use Swift.
==Edit==
This question here gives some background regarding .FBX usage.
Import FBX to ARToolKit
In short .FBX is not supported and one should use the OSG commandline tool to convert .FBX to .OSG
--- Personal note ---
From my experience is easier to use the Unity3D plugin of ARToolKit5 or ARToolKit6 do you animations with Unity3D and export an iOS app.