Unity can create lightmap UVs for you, and it's recommend that you do it through Unity as the process is much different than other external programs. Lightmap UVs in Unity tend to be pretty good so unless you have a very specific way of creating them externally I'd just let Unity do all the work. You can enable lightmap UVs when you import a mesh. It'll be a checkbox under the import settings. You can read more about it here: https://docs.unity3d.com/462/Documentation/Manual/LightmappingUV.html
