code:

#include "mainwindow.h"
#include "./ui_mainwindow.h"

#include <QProcess>
#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent), ui(new Ui::MainWindow)
{
  ui->setupUi(this);


}

MainWindow::~MainWindow()
{
  delete ui;
}


void MainWindow::on_pushButton_clicked()
{
  QProcess *proc=new QProcess(this);
  QStringList argv;
  argv.append(" enc -des-cbc -a -in C:/Users/Lincoln/Desktop/test.txt -pass pass:123");
  argv.append(" -out C:/Users/Lincoln/Desktop/test.enc");
  bool res=proc->startDetached("D:/OpenSSL-Win64/bin/openssl.exe",argv);
  if(res)
    QMessageBox::information(this,qApp->applicationName(),"Archivo encriptado");
  else
    QMessageBox::critical(this,qApp->applicationName(),"Hubo un error");
}

Note that I get an encrypted file, but below it says something else.