Is there any event when the size of Viewport changed?

Guys, I am trying to do some actions on vieport resize, but I am stopping in a “simple” error. I understand the delegate and the assign in C++ what I cannot find is the include to FViewport be identified and the program compiled.

Anyone knows? Can please tell me? I find the declaration in the “UnrealClient.h”, but it not work. Maybe is missing some string in build file?

I am using Unreal 5.3.

Thanks.