If you need to publish purchasing requirements on the Alibaba platform, you can follow the steps below:

1. Log in to the Alibaba website and enter the "Purchase" page.

2. Enter the product or keyword you need to purchase in the search box at the top of the page and click "Search".

3. In the search results page, select the product you need and enter the product details page.

4. In the product details page, click the "Contact Supplier" or "Purchase Now" button to enter the inquiry page.

5. In the inquiry page, fill in your purchasing requirements, including quantity, specifications, delivery time and other information, and leave contact information.

6. Click the "Send" button and your purchasing requirements will be sent to the supplier.

It should be noted that it is best to conduct market research before publishing purchasing requirements to understand the market price and quality level of the product in order to better grasp the market situation and save purchasing costs. In addition, it is also very important to choose the right supplier, which can be screened and compared through the supplier credit evaluation, transaction records and other information on the Alibaba platform.