After saving a form, go to Settings → Sharing where you can find Access management.
First toggle Allow only authenticated users and select Microsoft Account:
Once you turn it on - users will need to be authorized. New settings will become available to choose from.
Note
If you want to use the other option, check out our Azure AD B2C authentication article.
You can restrict submissions to just one per user. If you do - you can also allow users to go back to form and edit their submission (each edit counts as a submission and will trigger a flow/zap again):
Finally, you can restrict access to specific users by enabling Restrict access to specific users or groups (Azure AD) and inputting a Microsoft 365 domain:
This will require you to authorize the app to access domain and retrieve information about the users:
On top of that, you will also be able to select specific Azure Active Directory groups or even individual users to provide access to:
Users without permissions will not be able to access the form: