SharePoint online: Creating a new custom list form

Im creating an app that needs to customize the list form urls for each of:

Display Form New Form Edit Form

In the host web. I have a feeling I need to somehow create new forms for the content type as a forms designer tool i'm sampling does something similar. See my image below.

It seems the designer tool also contains a new forms application page which has a custom url.

PS. I don't want to use SharePoint designer. I need to do it through the existing API, assuming the JS API.

Thanks!