After you write your cover letter, take a look at both, the resume and cover letter. Make sure your cover letter isn’t simply a shorter version of your resume, because that would give the hiring manager the wrong impression of your understanding of what a cover letter is.
Take time to alter your cover letter as per your job application and let the hiring manager know why the job opening is important to you. The cover letter must show how your skills are aligned to the job role and how can you contribute to the company's vision.
How else would you get the hiring manager’s attention?