There are a few issues you need to be aware of when customizing an Outlook Appointment or Meeting form.

To open a form in Forms Designer in Outlook 2010 and Outlook 2013, you need to show the Developer ribbon.

Open a new form and click Design this Form or select a form from all forms by clicking Design a Form. When a form is in design mode, it will look similar to the screenshot to the right.

Need help designing a custom form? See Designing Custom Outlook Forms and Publishing Custom Forms in Outlook.

Issues when designing Appointment or Meeting forms

The first page of the Appointment form is not customizable. If you want to customize appointment form, you can hide the front page and design a custom page that looks like the first page.

Adding a control to a page (tab) will display that page by default. To rename or hide a page, check the options on the Page button.

Doing this leads to the next issues:

Outlook does not include a date and time picker. Outlook 2007 introduced date and time pickers for use with form regions. However, these are not designed for use on legacy forms. Using the new controls on legacy forms may cause form corruption and the loss of code behind the form.

After you publish an appointment form, when you open a new item with that form (other than by double-clicking in a Calendar folder's Day/Week/Month view), the Start date may default to the date you published the form. The form will also have a reminder set if the user's default option is to use a reminder. To avoid this problem you need to use code in the form that turns off the reminder and causes it to default to the current date.

Function Item_Open() 
  If Item.Size = 0 Then 
   Item.Start = Now() 
   Item.ReminderSet = False 
  End If 
End Function

You cannot use your own values in the Location drop-down list on the first page of an appointment form. Outlook maintains this list as a registry entry. See To clear the Location drop-down list on Outlook appointments

Ideas for custom Appointment or Meeting forms

Now that you know what to watch out for, there are a couple of useful uses for custom appointment forms:

Create one or more custom meeting form with the resources selected. If you always invite specific users to all meetings, add them to the form as well.

Set a field in every new appointment

When you open a new appointment form you can set a default value for any field using the Item_Open function in a script.

I'll demonstrate how to do this using a default setting that annoys a lot of people: Outlook's 30 minute duration when creating a new appointment.

Outlook's default duration is 30 minutes when you click the New Items button. To use a different duration you need use the day or week view with the desired scale then click in a time period. Or you can use VBScript in a custom form.

This simple script will set the default duration to 15 minutes when you click New Meeting or New Appointment command.

The basic steps to create a custom form are below. I have more information, including screenshots, at Designing Custom Outlook Forms and at Publishing Custom Forms in Outlook.

1. If the Developer ribbon is not visible, go to File, Options, Customize Ribbon and add a check next to Developer.
2. Click New Appointment.
3. Switch to the Developer ribbon and click Design This Form button.
4. Click View Code button.
5. Paste the Item_Open function into the Script Editor.
   

   Function Item_Open()
     If Item.Size = 0 Then
      Item.duration = 15
      End If
   End Function
   

6. Test it by clicking Run this form on the Developer ribbon.
7. Return to the forms designer and click Publish, Publish form as.
8. Type a name for your form and click Publish.
9. Set the form to be the default calendar form.

When you use Exchange server mailboxes, you can easily share the form with coworkers if it's published to the Organizational Forms Library. See Create an Organizational Forms Library for more information.