How to set an outlook custom form as the default form for a particular folder
If you want to change the default form for any folder except Mail folders, follows these steps:
1. Right click on the folder and click on Properties.
2. In the ‘General’ tab, select the form from ‘when posting to this folder use:’ field.
3. If the custom form (you want to set the default form) is published to the current folder, it will be listed in the dropdown you’ll need to ‘select Forms..’ and find it.
After making a your custom from as default form every time when you create a new item of that item type, Outlook will use the this custom form.