If you enabled the hidden built-in Administrator which is only to be used in emergencies, the correct method is shown here:
Enable or Disable Elevated Administrator account in Windows 10 | Windows 10 Tutorials
Then you would leave the password box blank to sign in with it.
What exactly do you want to do with this account. It is not your Admin account and cannot be used for anything except emergencies.
If you need an Admin account since you must always have one, and the PC owner ought to operate an Admin account for many good reasons, you can use the built-in Admin to create a new Local Admin account in Settings > Accounts > Family & Other People > Other, as shown here: http://www.howtogeek.com/226540/how-to-create-a-new-local-user-account-in-windows-10/
Sign into the new account, test that all programs work or else reinstall them, then when ready delete the old account by one of these methods:
https://www.wikihow.com/Delete-a-User-Account-Picture-in-Windows-10
Then hide the built-in Admin account again.
You can then change it to your Microsoft account if desired:
https://www.tenforums.com/tutorials/5375-switch-microsoft-account-windows-10-a.html
Hope it helps.