Handling the WM_CREATE notification in Win32+WPF scenario

[qmatteoq]

Hello, I'm working with a customer who is using WPF + Win32 interoperability to gradually modernize the UI of their application, by hosting WPF controls in a Win32 application.
They followed point by point list on how to go about embedding WPF content in a Win32 host available in the official documentation at https://docs.microsoft.com/en-us/dotnet/desktop/wpf/advanced/walkthrough-hosting-wpf-content-in-win32?view=netframeworkdesktop-4.8#the-basic-procedure.
Point 4 says the following: “Handle the WM_CREATE notification in your window procedure and do the following: ...”. Is it strictly required to perform these in the WM_CREATE message specifically? Or are they allowed to do this in messages that are sent later during the lifecycle of the host Win32 control?

Thanks!