dun know if it is 自荐. feel like that you can just write to agency and tell them what you want and if they happen to have vacancies (which happens all the time) and feel you are the right candidates, they will ask you for interview and recommend u to the company (if u are the lucky one lah)
most of the time, agencies will not charge the 求职者 but the company who hired them to employ people.
For ur case, if it is 有agent替某家公司招人，因不知对方具体的情况，所以很迷惑不知道怎样写cover letter。"
If you want to send a cover letter as well, u can just regard this agency as the company and then write the cover letter. Because by all means, agencies are helping the company to employe people. Cover letter shows more on your writting skills than resume, so a nice cover ltter can be helpful some time.
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