Hi All

I got the answer : In order to "Show fields of relation fields (advanced)" be checked to choose
the "fields/external_id" for Many2one fields. You should, in your file of importation csv or xlsx, make the header of the column like this : store_id / id  instead of store_id.

By using store_id / id => "Show fields of relation fields (advanced)" will be checked automatically and than you can choose external_id or whatever you want

Kind Regards

Put a slash "/" in a header (you way want to add fields name but it works without). No need to add datas in the column.