diff --git a/packages/javascript-api/src/lib/model/ticket/external-data.ts b/packages/javascript-api/src/lib/model/ticket/external-data.ts
new file mode 100644
index 00000000..e5c9977d
--- /dev/null
+++ b/packages/javascript-api/src/lib/model/ticket/external-data.ts
@@ -0,0 +1,34 @@
+export type ExternalData = {
+  fields?: ExternalDataField[];
+};
+
+export type ExternalDataField =
+  | ExternalMessageField
+  | ExternalTextualField
+  | ExternalListField;
+
+export interface ExternalMessageField {
+  type: 'message';
+  content: string;
+  importance: 'info' | 'warning' | 'error';
+}
+
+export interface ExternalTextualField {
+  type: 'text' | 'email' | 'phoneNumber';
+  title: string;
+  value: string;
+}
+
+export interface ExternalListItem {
+  title: string;
+  text?: string;
+  url?: string;
+  footer?: string;
+  timestamp?: string;
+}
+
+export interface ExternalListField {
+  type: 'list';
+  title: string;
+  items: ExternalListItem[];
+}
diff --git a/packages/javascript-api/src/lib/services/ticket/set-external-data.spec.ts b/packages/javascript-api/src/lib/services/ticket/set-external-data.spec.ts
new file mode 100644
index 00000000..d5bc3ee1
--- /dev/null
+++ b/packages/javascript-api/src/lib/services/ticket/set-external-data.spec.ts
@@ -0,0 +1,56 @@
+import * as sinon from 'sinon';
+import { Qminder } from '../../qminder';
+import { TicketService } from './ticket.service';
+import { ExternalData } from '../../model/ticket/external-data.js';
+
+describe('Ticket.setExternalData', () => {
+  let requestStub: sinon.SinonStub;
+
+  beforeEach(() => {
+    Qminder.setKey('EXAMPLE_API_KEY');
+    Qminder.setServer('api.qminder.com');
+    requestStub = sinon.stub(Qminder.ApiBase, 'request');
+  });
+
+  afterEach(() => {
+    sinon.restore();
+  });
+
+  it('sends POST to v1/tickets/<id>/external with stringified data and returns success', async () => {
+    const ticketId = '123';
+    const provider = 'crm';
+    const title = 'Case #42';
+    const data: ExternalData = {
+      fields: [
+        { type: 'message', content: 'Hello', importance: 'info' },
+        { type: 'text', title: 'Order', value: '42' },
+        {
+          type: 'list',
+          title: 'Links',
+          items: [{ title: 'Home', url: 'https://example.com' }],
+        },
+      ],
+    };
+
+    requestStub.resolves({ result: 'success' });
+
+    const result = await TicketService.setExternalData(
+      ticketId,
+      provider,
+      title,
+      data,
+    );
+
+    expect(result).toBe('success');
+
+    expect(requestStub.calledOnce).toBeTruthy();
+    const [url, options] = requestStub.getCall(0).args as [string, any];
+    expect(url).toBe(`v1/tickets/${ticketId}/external`);
+    expect(options.method).toBe('POST');
+    expect(options.body).toEqual({
+      provider,
+      title,
+      data: JSON.stringify(data),
+    });
+  });
+});
diff --git a/packages/javascript-api/src/lib/services/ticket/ticket.ts b/packages/javascript-api/src/lib/services/ticket/ticket.ts
index 0f4ff6a3..ef4dbe75 100644
--- a/packages/javascript-api/src/lib/services/ticket/ticket.ts
+++ b/packages/javascript-api/src/lib/services/ticket/ticket.ts
@@ -11,6 +11,7 @@ import {
 } from '../../util/id-or-object.js';
 import { ApiBase } from '../api-base/api-base.js';
 import { ResponseValidationError } from '../../model/errors/response-validation-error.js';
+import { ExternalData } from '../../model/ticket/external-data.js';
 
 /**
  * Represents a collection of search criteria for TicketService.count().
@@ -755,7 +756,7 @@ export function setExternalData(
   ticket: IdOrObject<Ticket>,
   provider: string,
   title: string,
-  data: any,
+  data: ExternalData,
 ): Promise<'success'> {
   const ticketId = extractId(ticket);