How to connect using PDB userid on Oracle 18c XE?
I have finally found a solution to my problem and I will explain to help other users in same case.

I have no problem to connect to XE instance of Oracle Database using following sqlplus command

sqlplus SYSTEM/password@XE


My first problem is that PDB is not always open and every time that I stop and start DB services, I must restart PDB.

To avoid this, I have executed following SQL command

alter PLUGGABLE DATABASE ALL OPEN;
alter PLUGGABLE DATABASE ALL SAVE STATE;


as explained on asktom.oracle.com

Now every time I stop and restart the Oracle database services, the VIRLUX PDB is correctly started but following sqlplus command continue to return an Oracle error that now is ORA-01017.

sqlplus test1admin/test1pwd@virlux

SQL*Plus: Release 18.0.0.0.0 - Production on Mar. Janv. 14 08:41:41 2020
Version 18.4.0.0.0
Copyright (c) 1982, 2018, Oracle.  All rights reserved.
ERROR: ORA-01017: invalid username/password; Logon denied


But, if I enter following command, I'm connected to Oracle PDB !

sqlplus test1admin/test1pwd@localhost/virlux


I think that old well known sqlplus connection command don't work for PDB and that the only solution is to prefix PDB name by hostname that has been defined in TNSNAMES.ORA file.

What is surprising is that a command that return an error's message saying that User/password are invalid is corrected without changind password !

This solution works now well on my PC on Windows Home 10.