If you need to manually enter a customer into ReCharge, there is an optional field titled Shopify ID when creating the record. It is not required, but depending on whether the customer has had orders placed through Shopify before, you may need to reference it.
If they have not, there is no need to put anything in that field.
If your customer has had orders placed through Shopify before, you need the records to correlate. You can easily locate this ID in your Shopify admin by going to Customers and searching and clicking on their name. The ID is located in the URL as the last digits