Hi,
this is not possible with a simple way, you need to override search form template override, so:
- Copy file /components/com_jsn/views/list/tmpl/default_search.php into /templates/your-template-folder/html/com_jsn/list/default_search.php
- Edit new file to met your requirement
In this post we have build an example that do this for you, try to copy attached file (unzip first) into /templates/your-template-folder/html/com_jsn/list/ folder (need to be created)
Another simple solution is to change title of field only for search forum (see screenshot)