add the value of the option to add if the option didn't exist

bin/behat-prepare.sh

@@ -82,7 +82,7 @@ terminus wp $SITE_ENV -- theme activate twentytwentythree
 terminus wp $SITE_ENV -- rewrite structure '/%year%/%monthnum%/%day%/%postname%/'
 # Add the pantheon-cache option if it doesn't already exist.
 if ! terminus wp $SITE_ENV -- option get pantheon-cache; then
-  	terminus wp $SITE_ENV -- option add pantheon-cache
+  	terminus wp $SITE_ENV -- option add pantheon-cache '{"default_ttl":600,"maintenance_mode":"disabled"}' --format=json
 fi
 # Set the starting default_ttl value to 600, our old default.
 terminus wp $SITE_ENV -- option patch set pantheon-cache default_ttl 600