How to integrate backend into sharepointlist

I want to create a form which I external users can access and backend is at (office 365)sharepoitnonline.

You could indeed embed your form in SharePoint, here is a guide that will provide you with the steps to do it: https://www.jotform.com/help/152-Adding-a-form-to-SharePoint 

Here's another link from Office Support that may help you adding the JotForm embed code: https://support.office.com/en-us/article/Display-a-Web-page-on-a-SharePoint-page-by-adding-the-Page-Viewer-Web-Part-7f61feec-9b3d-4805-a960-07636ba59527?ui=en-US&rs=en-US&ad=US&fromAR=1 

Hope this helps.