How can I know if any company has posted it? If so, how can I get in?

Open Baidu's homepage, then click the post bar button to enter the post bar interface, then enter the company name in the search box, and then click Baidu. If there is, you can enter their post bar. If not, you will be prompted to create one or search for the name of their company on Baidu. If there is, it should be on the first page of search results.