Save user id of web application in MySQL audit table

I have the following TRIGGER which inserts certain specific data for the purpose of auditing. I need that in the field Audit user the id of the user who started the session in the system and not the one in the database is saved.

DELIMITER //
CREATE TRIGGER tr_up_salario
  AFTER UPDATE ON empleadocargo
  FOR EACH ROW
  BEGIN
    declare _user varchar(20);
    declare _idD, _idA int;

    set _user = (select USER());
    set _idD = NEW.idEmpleadoCargo;

    insert into rrhh.auditoria (usuarioAuditoria, accion, tabla, registroNro)
    VALUES (_user, 'UPDATE', 'EmpleadoCargo', _idD);

    set _idA = (LAST_INSERT_ID());

    insert into rrhh.detalleauditoria (Auditoria_idAuditoria, nombreColumna, antiguaDescripcion, nuevaDescripcion)
    VALUES (_idA, 'Salario', OLD.salarioFijo, NEW.salarioFijo);
  END //
DELIMITER ;

How do I access the user id and insert it into the table from the TRIGGER?