very basic way of sending a reaction

src/domain/session/room/timeline/tiles/BaseMessageTile.js

@@ -122,6 +122,10 @@ export class BaseMessageTile extends SimpleTile {
         return this._reactions;
     }
 
+    react(key) {
+        this._room.sendEvent("m.reaction", this._entry.annotate(key));
+    }
+
     _updateReactions() {
         const {annotations} = this._entry;
         if (!annotations) {

src/matrix/room/timeline/entries/BaseEventEntry.js

@@ -16,6 +16,7 @@ limitations under the License.
 
 import {BaseEntry} from "./BaseEntry.js";
 import {REDACTION_TYPE} from "../../common.js";
+import {createAnnotation} from "../relations.js";
 
 export class BaseEventEntry extends BaseEntry {
     constructor(fragmentIdComparer) {
@@ -80,4 +81,8 @@ export class BaseEventEntry extends BaseEntry {
             // so don't clear _pendingRedactions here
         }
     }
-}
+
+    annotate(key) {
+        return createAnnotation(this.id, key);
+    }
+}

src/platform/web/ui/session/room/timeline/BaseMessageView.js

@@ -99,6 +99,7 @@ export class BaseMessageView extends TemplateView {
         } else if (vm.canRedact) {
             options.push(Menu.option(vm.i18n`Delete`, () => vm.redact()).setDestructive());
         }
+        options.push(Menu.option(vm.i18n`React with 👍`, () => vm.react("👍")))
         return options;
     }