Dear Alec217 ,
Thanks for choosing Microsoft community!
I have read your question carefully and done some research, please kindly refer to following:
First, go to File, Options, Customize Ribbon and add a check to Developer ribbon on the right.

Step 1: Go to the Calendar folder
Open the calendar folder you want to publish the form in. This makes it easier to publish the form because it gets published to the folder you are "working" in.
Step 2: Create custom appointment form
Open a new appointment form and change Show as time as desired.

Step 3: Publish the form
Publish the form using the Publish form command on the Developer ribbon

The default location should be calendar so you just need to enter a name for your form.

Step 4: Set the new form as default for new appointments
Right click on the calendar folder, choose Properties and select the form from the dropdown to set it as default.

Now when you create a new appointment, the Free/Busy time will default to your desired setting, Free in this example.
Hope this help you, if there is any deviation in my understanding, please forgive me and provide me with more details. I am more than happy to continue to help you. I wish you all the best!
Best regards,
Kerry