You can redeem a discount code at airalo.com or on the Airalo app.
On the Airalo website:
Go to Airalo Website >>
Log into your Airalo account >> 
Select an eSIM >>
Click “BUY NOW”>> 
Click Redeem Discount Code at the checkout page >>
Enter the discount code and click Redeem >> 
On the Airalo app:
Open the Airalo App >>
Log into your Airalo account >>
Select an eSIM >>
Tap “BUY NOW”>>
Select “Apply Code/Use Airmoney">>
Enter the code and tap “Apply”
If you have any questions, please feel free to contact support.