Short answer
Publish the form as a public web form with its own URL. The person filling it in needs no Microsoft account and no sign-in, and the submission still writes into your SharePoint list. Forms 365 Web does this directly. The forms SharePoint builds from a list do not, because they inherit the site's permissions.
Why the built-in list form cannot do it
When you make a form from a SharePoint list, that form lives behind the site. To open it, a person has to authenticate against your tenant and have permission to the list. That is correct behaviour for staff. It is a wall for a customer, a contractor, or an applicant who has no account with you and never will.
The usual patches do not really solve it. Microsoft Forms takes anonymous responses, but they land in its own storage, not your list, so you are back to a Power Automate flow. The Request Files feature collects documents from outsiders, but only files, with no form fields around them. None of these gives you a proper form that writes structured data into a SharePoint list from someone with no login.
The public web form approach
Forms 365 Web is built for the external case. You design the form, publish it to a public URL, and share that link or drop it behind a QR code. Anyone can open it and submit, no Microsoft account involved, and each submission is written into a SharePoint list in your own tenant. The data lands where your team already works, so your views, permissions, and any flow on that list carry on as normal.
When you know who you are inviting
Sometimes the recipient is not anonymous, you just do not want them signing in. For that, Forms 365 Web has single-use invitation links. Each person gets their own tokenised link, so you can pre-fill what you already know about them and keep one submission tied to one invite. It suits supplier onboarding, renewals, and anything where you are chasing named people rather than opening the door to the world.
A note on the data
Nothing about going public changes where the data lives. Submissions are ordinary SharePoint list items in your Microsoft 365 tenant. You are not shipping customer data into a separate product to get a form that outsiders can use.
Open one form to the outside world.
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