Most scheduling applications like Calendly, Setmore, or Doodle provide a link that you can provide to clients so they can access yor calendar and set an appointment.
The simplest way to add appointment scheduling to your app is to use a link action that points to the link provided by your calendar appointment application. Add the scheduling link to a link action and an effective call to action and yor are done.
The other option is to add the appointment scheduling as an embed. Take Calendly as an example. After you have set up your account, you will see an option to "</> Add to Website" under the "Copy Link" option.
Choose the "Add to Website" option and then the "In-line Embed" option which will display a page where you can get the embed code, as in the image below:
Use the "Copy Code" button on this page to copy the needed text and then paste it into the an "HTML" action in your app.
It may not be sized right for a mobile device, so you will need to edit the two numbers highlighted in red above to size the display box correctly for your app. These numbers are the horizontal width and vertical height of your embed. The width should typically be "180", but the correct height will vary, depending on how many event choices you allow as appointment options.