Create a Purchase as you normally would.

Create a Vendor Bill for the amount you need to prepay using an expense or prepaid account as applicable (1) and then click the new Purchase Matching button (2):

From the screen you are taken to, click the line you just made and at the top click Add to PO (3) and then Add Down Payment (4) once you have selected the correct Purchase Order:

This will add a new line to your Purchase Order:

When the next Vendor Bill is created, the down payment will be added to the bottom:

If you only want to use half or a quarter of your down payment on a given Bill, adjust the quantity (still keep it negative) to 0.5 or 0.25 as required.

Thank you very much for your reply. In fact, I am a bit confused about the prepaid logic and operations in Odoo 18. it is more better if it can perform prepaid operations directly on the purchase order view. how to set if I want operate it like previous version?