How to set up a URL constant in WordPress

I have a client’s website where they have several occurrences of an event URL across the site. Unfortunately, they have just told me that this URL changes from month-to-month. To save my client having to edit every occurrence of that URL, I am wondering if there is a plugin or a WordPress trick where we can set up a “URL constant or variable” and link to that everywhere and then change the value behind the URL constant or variable in the dashboard. ie. A bit like an include file but it needs to be managed by the client from the dashboard. They update the URL once in that special location and the system uses it wherever the URL constant or variable has been used in the website.

Thanks in advance.