From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu in which you can select 'Packages'.
And click 'Vouchers' from the 3 options at the top of your page.
You can then start creating your discount voucher by hitting the 'Create Voucher' button at the bottom of your screen
Fill in all the details in regards to how much discount the voucher will offer
And finish by clicking the 'Create' button.