Okay, so you need a custom authentication schema then.. Using FND_WEB_SEC.validate_login maybe..
Ex:
FUNCTION ebs_authenticate (
p_username IN VARCHAR2
, p_password IN VARCHAR2) RETURN BOOLEAN
AS
BEGIN
RETURN (FND_WEB_SEC.validate_login
(p_username, p_password) = 'Y');
END ebs_authenticate;
END IF;
END;
Please check the following doc ->
https://www.oracle.com/technetwork/developer-tools/apex/apex-ebs-wp-cabot-consulting-169064.pdf