You may get username, password or connection_string parameters' values for internal oracle database authentication screen by substituting them as argument of get_application_property method.
ON-LOGON and ON_ERROR triggers at form level may be used for you to manage your task :

where ON-LOGON's code text is :
declare
v_cnn varchar2(35):='myDB';
begin
:global.v_lng := 6 ;
:global.v_pwd := get_application_property(password);
if ( length(:global.v_pwd) > :global.v_lng ) then
logon('myschema',:global.v_pwd||'@'||v_cnn);
end if;
end;
ON-ERROR's code text is :
begin
if DBMS_Error_Code = -1017 then
:global.v_pwd := get_application_property(password);
if ( length(:global.v_pwd) < :global.v_lng ) then
message('The length of the Password should be at least '||:global.v_lng||' characters !'); message('');
else
message(DBMS_Error_Text); message('');
end if;
end if;
end;
as an authentication example with certain minimum length for a schema's password.