ASP.NET Core 2.1 Identity couldn't find ApplicationUser

Question

I've included Identity into my .NET Core2.1 project, when I try to build the project, it warns me with the following error:

The type or namespace name 'ApplicationUser' couldn't be found (are you missing a using directive or an assembly reference?)

Should I define ApplicationUser myself or Identity will create it by itself?

any help would be appreciated.