It is described in the main help file under ClearWin+ > Format code reference >W >%wb.
Here is a simple example...
program WebPage
use clrwin
integer iw
integer,external::cbForum,cbWeb
iw=winio@('%ww[no_border]%ca[New HTML]&')
iw=winio@('%pm[Back,Forward,History,Copy,Forum]&', &
& 'wbGoBack', 'wbGoForward', 'wbHistory', 'wbCopy', cbForum)
iw=winio@('%pv%60.30^wb[http://www.silverfrost.com]', cbWeb)
end
integer function cbForum()
use clrwin
integer r
r = wbCommand@('http://forums.silverfrost.com')
cbForum = 2
end function
integer function cbWeb()
use clrwin
character(len=256)::reason,url
reason = clearwin_string@('CALLBACK_REASON')
url = clearwin_string@('CURRENT_URL')
print*, reason(1:len_trim(reason)), ' = ', url(1:len_trim(url))
cbWeb = 2
end function