Spoiler alert: a letter of intent is not the same as a cover letter. They're similar (being letters and all, and focused on yourself), but are actually used in different situations. Your cover letter is what you write when you're applying to a specific job you found through traditional channels (online job search, referral, recruiter). It details why you're a great fit for this particular job. A letter of intent is what you write when you're cold-calling (leaving a resume without being solicited for one), or applying for a job in a more general situation, like a job fair or submitting your resume to a general pool. The letter of intent is similar in that you're selling yourself, but tends to be less granular about a particular position. Letters of intent are often more networking-related, or aspirational, than position-oriented. Because you may have fewer specifics in hand about what you're applying for and who will be reading your application package, it's likely you won't have the most personalized opener. That's okay! Be general, but professional, formal, and polite.
Here's where the difference comes in between a cover letter and a letter of intent. With a cover letter, you likely already have a solid idea of what the job opportunity is, and how to position yourself for it. With a letter of intent, you have to make a slightly trickier balance—positioning yourself as qualified for a job that may not be clear yet. To do that, align your self talk around the company or the industry, making sure to highlight your skills and achievements that would make you a good fit for the company. You can also be specific about your level and experience. For example, you should make it clear that you're looking for a manager-level position if you don't want to be considered for more junior or entry-level roles.