Add User Custom Action via /_api/site/UserCustomActions

Hey, all!

I have been scouring the web to figure out how to add a User Custom Action by using the /_api/site/UserCustomActions endpoint.  I can find JSOM and CSOM examples, but I need this to work via REST.  Ideally I need an example in JavaScript, but I should be able to convert between almost anything.  I'm still on SharePoint 2013, but this solution should work all the way through SPO.

Here are a couple examples I've found that did't quite give me what I need (proof I've actually searched for this).

This example pulls the userCustomActions via REST, but adds them through JSOM
https://github.com/johnnliu/UserCustomActionsConfigPage

This is an example of how to delete a userCustomAcion via REST
https://www.c-sharpcorner.com/article/sharepoint-online-delete-custom-action-via-rest-api/

Has anyone used this REST feature and have code to share?  Thanks in advance!

Jason Thompson