How can I add a widget made by me to my form?

Have you published the widget here? If yes, then you should be able to see your custom widget on the widget page. You can add it to your form.

Step 1, Create the widget and fill the form

Step 2, Register the widget

Step 3, The widget would be listed under "my widgets".

It will require approval from the widget/API team once you publish it. I suggest you send your inquiry directly to our API team here api@jotform.com

Thanks.