code: public void setSiteParam(String siteRoot, String key, String value) {
  
          CmsSite site = OpenCms.getSiteManager().getSiteForRootPath(siteRoot);
          if (site == null) {
              throw new IllegalArgumentException("No site found for path: " + siteRoot);
          } else {
              site.getParameters().put(key, value);
              OpenCms.writeConfiguration(CmsSitesConfiguration.class);
          }
      }
docs: Sets a site parameter and writes back the updated system configuration.<p>
  
  @param siteRoot the root path used to identify the site
  
  @param key the parameter key
  @param value the parameter value