When you submit your resume, you can say that there are your job search records online, and companies have their panels, just like you can see your basic information when you search for a position. For you, sending a cover letter to someone else's mailbox will attach the resume on the website as the main body, and your resume can be used as an attachment.

The subject of the mailbox is xxx's cover letter. I suggest you write one yourself if you send it. There is no need to use things on the website. You just need to know someone else's email address.