If you've logged in recently, you'll be asked to confirm which account you want to sign in under. If it's the account you want to manage your new company with, click "Yes, that's correct.".
If you haven't signed in recently, you'll be asked to create an Intuit account. When you see this, scroll down and find your Adding a company to an existing account? option, then select Sign in. When prompted, enter the same login credentials for your existing account that you want to add this new one to.
3. Fill out all of the appropriate information about your company and finish setting up the new subscription.
After completing this process, you'll be able to switch between your companies by using the Gear (⚙️) icon and selecting Switch company.
I've included a couple resources about adding new companies to an existing one and switching between them that may come in handy moving forward: