Instead of typing and if you already have it in Excel you can also copy and paste it in the “Enter data” window as long as you aren’t pasting more than 3000 “cells” ( I believe ).
Alternatively you can also get your fact table into Power Query.
Right click on the Column header with your Customer names and select “Add as New Query”
On the Home tab select “Remove Rows”, “Remove Duplicates” and
On the Add Column tab select “Index Column”, “From 1”
That way new clients will automatically be assigned an Index number BUT the same is true if there’s a typo in the Customer name of course… with a bit of work you can have PQ check for that as well