Well, when I read your initial post and a comment, it’s still unclear where you want that base_url to be in your template. Maybe it’s a good idea to articulate your goal more precisely? 
To answer your question as I understand it now - you need to create an input_text:
input_text:
base_url:
initial: !secret base_url
and then in your template
{% set url = states('input_text.base_url') %}