You need the purple window in the right corner of the page, the Buy USDT (TRX) section.
In the first field you need to choose your desired currency. Click the arrow and choose from the dropdown list.
In the second field ("I want to buy" / USDT), choose Tether (TRC-20) USDT * Tron from the list. Please double check and make sure you chose Tether (TRC-20) USDT exactly!
In the second field (USDT), type the desired amount of crypto you want to buy (the price of the product you want to pay for). Press "Continue".
Indicate your email adress, press "Continue".
Choose the payment method you prefer by clicking on one of them.
Fill out the form with your basic details (name, age, date of birth). Press "Continue".
Fill out the billing address form, press "Continue".
Enter your card details, press "Continue".
Enter the Tron wallet address. Please double check to avoid any mistakes! Press "Continue".
Read carefully the points and press "Yes, it is safe to use".