Solution: I had
PublicAdditionalLibraries.Add("User32.lib");
in my Build.cs file, but actually it was not loaded because for some reason UBT was not able to find it. So I just used the full path like:
PublicSystemLibraries.Add("C:\\Program Files (x86)\\Windows Kits\\10\\Lib\\10.0.26100.0\\um\\x64\\User32.Lib");