How to use a substitution variable in Toctree Sphinx?

Question

The problem consist to use a substitution variable var_link which is declared in conf.py file as follow :

var_link = 'https://www.google.fr/'

This variable is added into rst_prolog by

rst_prolog = f"""
.. |var_link| replace:: {var_link}
"""

And I need to use this link in my toctree index as :

.. toctree::
   :maxdepth: 1

   Title <|var_link|>

When I try the above syntax, it gives me a warning :

WARNING: toctree contains reference to nonexisting document '|var_link|'

Thanks for your helpful