How to add a customer tag to a list of contacts

Hi Gaspard, 
You should create a flow that would be triggered by your list. You’ll then have to add the action “Update profile property”
 

Choose “Create New Property” with for example the label “Customer” and the value “True”.

Finally, you’ll have to back-populate the flow, and every profiles in this list will be tagged.

I hope this was helpful. :)

Feel free to send me a private message to schedule a call if you need help or want to optimize your Klaviyo account.

Cheers!