Repeating table in a form – Sharepoint list – Help needed

Dear,

I am trying to insert a repeating table to an existing form that is connected to a Sharepoint 2013 list

There’s no way to insert a repeating table since the option “automatically create data source” is greyed out. 

My intention is to insert a table that allows the users to insert rows as needed when filling, like:

Product description                Size                 Value

Any ideas for a solution?

Thank you!