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How do I write a general cover letter for a job application website? - cover letter hygiene

I want to send my resume to a website that lists many jobs that could be classified, too. This site allows you to make your resume and cover letter, and then click on the specific task you want to apply. All my previous entries were directed against a particular point - and in line to write letters. How to write a letter showing my strength properly, without a specific vocational orientation or position of the employer?