Found the solution! Here it is, if somebody needs the description.

The webview_bind() function in DLL that expects to get a pointer to the function in Pharo can be called like this:

WebViewLibrary >> bind: webview name: aString callback: aFFICallback arguments: aByteString

	^ self ffiCall: #(void webview_bind(void * webview, const char * aString, FFICallback aFFICallback, void * aByteString))

Before doing the actual call, the callback should be made according to fn signature:

doIt := FFICallback 

		signature: #( void (const char *seq, const char *req, void * arg) )

		block: [ :seq :req :arg | 

                        ���do the stuff���

		].

And the call is then made as:

WebViewLibrary new bind: webview name: 'newJsFunctionName' callback: doIt  arguments: args.

The next challenge is, of course, threading ...

Best wishes,
Tomaz