To include a forgotten password form in your theme, you just need to create an HTML form with its action attribute set to /account/forgotten-password
Within this form you should have an input of type text with a name attribute set to to email
An example is shown below.
<form action="/account/forgotten-password" method="post"> <input type="text" name="email"> <button type="submit">send</button> </form>
Once the form has been submitted the user is redirected to the account/login view.
A new password will then be emailed to the user.