DECLARE
    l_token_url      VARCHAR2(32767) := 'https://cacm-api-stage.ascend/auth/v2/login';
    l_client_id      VARCHAR2(32767) := 'ex*********vc';
    l_client_secret  VARCHAR2(32767) := '20*********J';
    l_flow_type      VARCHAR2(32767) := 'OAUTH_CLIENT_CRED';
    l_wallet_path    VARCHAR2(32767) := 'file:' || CCM_INT_API_PKG.GET_DB_DIRECTORY_PATH('DATA_PUMP_DIR');
    l_access_token   VARCHAR2(32767);

    l_vcHeaderName  VARCHAR2(32767);
    l_vcHeaderValue VARCHAR2(32767);
BEGIN
    APEX_DEBUG_MESSAGE.ENABLE_DEBUG_MESSAGES(p_level => 4);

    BEGIN
        apex_web_service.oauth_authenticate(
            p_token_url      => l_token_url,
            p_client_id      => l_client_id,
            p_client_secret  => l_client_secret,
            p_flow_type      => l_flow_type,
            p_wallet_path    => l_wallet_path
        );
    EXCEPTION
        WHEN OTHERS THEN
            DBMS_OUTPUT.put_line('Error: ' || SQLERRM);
            RETURN;
    END;

    DBMS_OUTPUT.put_line('Response Headers: ');
    FOR i IN 1.. apex_web_service.g_headers.count LOOP
       l_vcHeaderName := apex_web_service.g_headers(i).name;
       l_vcHeaderValue := apex_web_service.g_headers(i).value;

        DBMS_OUTPUT.put_line(l_vcHeaderName || ' : ' || l_vcHeaderValue);
    END LOOP;

    l_access_token := apex_web_service.g_oauth_token.token;

    DBMS_OUTPUT.put_line('Access Token: ' || l_access_token);
    DBMS_OUTPUT.put_line('Status Code: ' || APEX_WEB_SERVICE.G_STATUS_CODE);
    DBMS_OUTPUT.put_line('Reason Phrase: ' || APEX_WEB_SERVICE.G_REASON_PHRASE);

    APEX_DEBUG_MESSAGE.DISABLE_DEBUG_MESSAGES();
END;

Output:
---------------------------------
Error: ORA-20001: Authentication failed.

Statement processed.

1.11 seconds