Add installment payment on OpenCart

To add installment payment on OpenCart 4:

1. Log in to the Merchant Back Office Systempay with your identifiers :
   https://paiement.systempay.fr/vads-merchant/
2. Go to Settings > Shop.
3. Click the Keys tab.
4. Copy your Shop ID.
5. Open another tab in your browser.
6. Sign in to your OpenCart Back Office.
7. Click on Extensions menu.
8. Click the Extensions sub menu.
9. Select the Payments Extension type.
10. Search for Systempay - Payment in installments and then click on
11. Search for Systempay - Payment in installments and then click on
12. Paste your Shop ID to the Access to the payment gateway.
13. Return to the Merchant Back Office.
14. Copy your Test key.
15. Return to the OpenCart Back Office.
16. Paste your Test key to the Access to the payment gateway.
17. Return to the Merchant Back Office.
18. Copy the production key.
19. Return to the OpenCart Back Office.
20. Paste your key to the Production key parameter.
21. Change the Mode parameter to PRODUCTION.
22. Click the Payment in installments tab.
23. Enter the percentage of the first payment in the First installment parameter.
    Example: Enter 50 for 50%.
24. Enter a total number of due dates in the Number field.
    Example: 3
25. Enter the time in days between two due dates via the Period parameter.
    Example: 30
26. To save your changes, click on