You will need code that looks something like the following. You will need to attach the D2KWUTIL.pll library to your form and ensure D2KWUT32.dll is in your ORACLE_HOME/BIN or windows system directory.
PROCEDURE call_url (p_url IN VARCHAR2) IS
l_browser VARCHAR2(500);
BEGIN
l_browser := Win_Api_Environment.Read_Registry('HKEY_CLASSES_ROOT\http\shell\open\command',NULL,TRUE);
--
l_browser := Replace(l_browser,'%1',NULL);
l_browser := Replace(l_browser,'%2',NULL);
l_browser := Replace(l_browser,'%3',NULL);
l_browser := Replace(l_browser,'%4',NULL);
l_browser := Replace(l_browser,'%5',NULL);
l_browser := Replace(l_browser,'%6',NULL);
l_browser := Replace(l_browser,'%7',NULL);
l_browser := Replace(l_browser,'%8',NULL);
l_browser := Replace(l_browser,'%9',NULL);
l_browser := Replace(l_browser,'""',NULL);
Win_Api_Shell.WinExec(l_browser || ' ' || p_url);
EXCEPTION
WHEN OTHERS
THEN
Message('Unable to find browser');
END;