Subscribe customer in specific segment to a list via flow

Hi,

I have a shopify markets account with multiple shops (URLs) under it. When someone checksout, they are automatically added to email marketing (unless they uncheck this themselves). Now I would like to add these customers to the general newsletter list (divided by country). As these are multiple lists, I cannot link this to one particular list via intergrations. 

I have already created segments so that everyone who accepts marketing is divided by country. Now I would like to create a flow where I indicate that when a customer accepts email marketing via the checkout, they are added to the general email list. Unfortunately, I cannot find this option when setting up a flow. Is there another way to automatically add these customers to the general list? 

Thank you in advance!