Based on your description, I understand that accounts and passwords of users are stored in the cloud, you want to create a form in the SharePoint Online to validate accounts and passwords of users, if the validation is successful, then the record will be added into a excel file.
Per my knowledge, above requirement is not available in the SharePoint Online.
As a workaround, you could create a Microsoft form to register user information. Here're steps:
1.Create a page.
2.Add the Microsoft form web part in the page -> New form -> Enter name -> Create.
3.Add new questions.
4.After users have submitted form, you could see responses in the excel file.
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