set the client_extras_dir

config.vapi

@@ -0,0 +1,4 @@
+[CCode(cprefix = "", lower_case_cprefix = "", cheader_filename = "config.h")]
+namespace Config {
+	const string PLUGIN_DIR;
+}

meson.build

@@ -24,6 +24,11 @@ dependencies = [
 	meson.get_compiler('c').find_library('m')
 ]
 
+config = configuration_data()
+config.set_quoted('PLUGIN_DIR', join_paths(get_option('prefix'), get_option('datadir'), 'xi-gtk', 'plugins'))
+configure_file(configuration: config, output: 'config.h')
+dependencies += meson.get_compiler('vala').find_library('config', dirs: meson.current_source_dir())
+
 sources += import('gnome').compile_resources('resources', 'resources/gresource.xml', source_dir: 'resources')
 executable('xi-gtk', sources, dependencies: dependencies, install: true)
 configuration = configuration_data()

src/Application.vala

@@ -63,7 +63,7 @@ class Application: Gtk.Application {
 		});
 		string config_dir = GLib.Path.build_filename(GLib.Environment.get_user_config_dir(), "xi-gtk");
 		GLib.DirUtils.create_with_parents(config_dir, 0777);
-		core_connection.send_client_started(config_dir);
+		core_connection.send_client_started(config_dir, Config.PLUGIN_DIR);
 		unowned string theme = GLib.Environment.get_variable("XI_THEME");
 		if (theme == null) {
 			theme = "InspiredGitHub";

src/CoreConnection.vala

@@ -150,9 +150,10 @@ class CoreConnection {
 		send_request("edit", params, response_handler);
 	}
 
-	public void send_client_started(string config_dir) {
+	public void send_client_started(string config_dir, string client_extras_dir) {
 		var params = new Json.Object();
 		params.set_string_member("config_dir", config_dir);
+		params.set_string_member("client_extras_dir", client_extras_dir);
 		send_notification("client_started", params);
 	}