Hello,
If that admin user was first set up as a local user, and later linked to a Microsoft account, the password would become the Microsoft account's password, not the one you set up.
So, I assume you currently sign into a user that doesn't have admin right?
In this case, I am afraid there's no way to go around it, nor to find the password, unless you know the password, or know what account it has been associated with, so you can reset it via https://account.microsoft.com.
When you first launch Windows, on the sign in page, do you see the admin user listed?