how to construct a dynamic url using template manager ?

Hello,

I followed your tutorial on how to “build API-URL with Use Custom Fields” and it works fine.
https://api.json-content-importer.com/build-api-url-with-use-custom-fields/

It uses shortcode set up in a page but I would like to use a template instead.
Could you tell me what would be the syntax for making the same example work via the template manager; ie: what should be the url, what should be in urlparam or curl options ?

thanks for your help.