I had to create ticks and subticks with QCPItemLine. Here is the code:
for(int i=-65; i<=65; i+=10)
{
// Ticks
QCPItemLine *tickHLine=new QCPItemLine(ui->customPlot);
ui->customPlot->addItem(tickHLine);
tickHLine->start->setCoords(i,-1);
tickHLine->end->setCoords(i,1);
tickHLine->setPen(QPen(QColor(137, 140, 140), 1));
//Subticks
QCPItemLine *subTickHLine=new QCPItemLine(ui->customPlot);
ui->customPlot->addItem(subTickHLine);
subTickHLine->start->setCoords(i+5,-2);
subTickHLine->end->setCoords(i+5,2);
subTickHLine->setPen(QPen(QColor(137, 140, 140), 1));
}