Access WCF(Azure or On-Prem) from SharePoint online(JQuery or Javascript)

The main goal is to expose SharePoint online form data in SQL tables.

1. The form in SharePoint online is in JavaScript/JQuery/HTML
2. Upon submitting the data should store in SQL tables(On premise)
3. For this we've written a WCF service(both Azure and On premise) and are trying to access the WCF service from SharePoint online form

Any help regarding this would be helpful.