how do I add custom post fields?

In other words, for each listing, there are default fields that customers can input information, such as Listing Title, address, phone number, etc. However, I’d like to add a field for “Company Name and Designation, where customers would input the value for those custom fields. I would like these by default to show up for each listing that a customer signs up (and pays) for.

I see within the WP Admin, that I can individually for each listing I can edit the listing to add a custom post field, but that’s not what I want it to do. Again, I want these custom post fields to already be there when customers input/complete their own listings. On a side note, in the edit a listing as WP admin, the add a new custom field asks me for a value which I don’t know what to put in anyway.