How to add custom column referring to serial number in a web form

Summary:

Client has a requirement, to have a first column as serial number in a web form, how to add a custom column, as all present dimension are been used, so can't disturb any dimension hierarchy.

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):