Where I can see log of Web Shield?

Visiting a page, when I clicked a button it oppened a new window with add, but in fact was malverstisment and Avast blocked it as a threat.

Now how I can find the log to inform about the fraud add page to the webmaster?

It depends on the OS you are using.
Here are some possible locations :
C:\Documents And Settings\All Users\Application Data\Avast Software\Avast\Log
C:\ProgramData\Avast Software\Avast\Log. Some may also be in the respective \Report folder.
C:\Users\All Users\AVAST Software\Avast\log

Windows 10

Also I asked if within Avast interface can be seen, as I cant found it ar interface

C:\ProgramData\AVAST Software\Avast\log

And then? What file? there are several .log

The file you actually want is C:\ProgramData\AVAST Software\Avast\report\WebShield.txt, this is where the web shield alerts are retained.

See image extract of the web shield report file.

thats why we need a complete “notification” details report of history blocked threat

https://forum.avast.com/index.php?topic=196739.0

Well we used to have a Real Time Stats section, but that was withdrawn for a redesign, made a brief return only to be withdrawn again. I felt that the redesign offered even less information than the original.

It would be nice if there was at least a shortcut to the report file or do something like the AvastUI > Protection > Antivirus. That however, would take a major redesign rather than just pointing to the report file.