My hunch would be to do the following:
1) Add a text area to your Post settings via the Theme settings API, and perhaps a switcher.
2) In the index.hbs file of your site’s files: go to the part of the code where a post’s link is set. Use Handlebars logic to indicate that if the switcher in the Post’s settings is toggled on, use whatever text input as the link URL instead of the post URL.
3) When you create a post that you want to redirect elsewhere, enable the toggle in Publii, and paste the URL in the text area.
You may need to browse through the dev docs, but that’s what I assume could work.
