template.php

Hi,
I will start with that I am making a webpage for car dealer. They have many cars listed in one popular car sales website https://eng.auto24.ee/main/mainindex.php the website has this system that connects all this dealers cars with their page that I am creating so that they only have to edit and update the sales ads in one place. So system Works then if i make Tepmlate page in my server and send them the URL. Only they can see it . I need to place {AUTO24_CONTENT} and they replace this string with the list of cars and I should be able to change my tmplate design. Then they give me the url for the page (my page that i created but they have added the list there) and I have to redirect my car sales page to the url where they have put the list.

All of the above I have done, I have put the content string on my sales page in the text editor and sent them the url, but they said they cant use it because they cant download string from the same page where they would add the list, they cannot have circular effect in their server.

Then I created a new page on my site naming it Template and added the string in text editor, that also didnt do it, because they needed the string to be in html source file, and I didnt know what it means.

I am sorry if this doesnt make any sense but I dont know who to ask anymore. The car sales site developers cant give direct answers to my questions and they dont have time to book me in for consultation. Also this is my first wp site.
Thanks in advance for any help.