I just want to preface this with a disclaimer. There are a lot of users that know way more about this stuff than I do. My statements on the matter are based on what I know.

Unless you are talking about an editor other that the 2017 source mode or the 2010 editor, I don't think a fix will be that simple. I don't know the script too well but based on a quick look, it appears to rely on identifying the selected area in the text field of the editing form and then inserting the text. However, both new editors appear to work quite differently. It seems that they wrap each and every line in a bunch of container tags that are all children of what visually appears to be a text field; but it isn't. Each line is its own subtree in the DOM tree.

I tried to see if simply copy-pasting a line in the DOM would register as added content. In the 2017 editor, it doesn't display the added line and the added line appears to interfere with line identification. Let's say I insert a duplicate of line 1 as the new first line using my browser's dev tools. As I run my cursor over the elements in the element inspector, it should highlight them on the page. The new line 1, despite not being displayed, highlights the old line 1. The old line 1 highlights the old line 2; and so on. The 2010 editor does not have this issue. The inserted lines display and are associated correctly. However, in both cases, the inserted lines apparently don't register with whatever is responsible for saving the edit. I try to save the changes and no change is actually made.

So it appears that there is some other program running in the background keeping track of each line and that its content is what actually gets saved. Unless and until it is clear what that program is and there is a way for site JS to interact with it, then it seems unlikely PreloadTemplates can be adapted.

Dammit, why it is so complicated now... I also saw that the structure of the edit area was more complex now. I did not understand what you did when you said when you "tried to see if simply copy-pasting a line", because I thought that maybe a workaround was to see if it was possible to copy the template syntax to the clipboard and then paste it, rather the directly insert it at the cursor position. Is that what you tried? It seems you did something with the browser dev tools instead.

This is really a bummer, this script saved me a lot of time. Btw, is this the right place to ask for help to updated scripts to UCP? There are other scripts that I wish to update, but don't know how.

Well, like I said. I am far from an expert on this stuff. I was just telling you what I observed hoping it would help. As it turns out, I don't understand the legacy editor either. Everything I read online says there should be text between the textarea tags but there isn't. So clearly there is something complex going on there as well.

That said, I'll still clarify what I did. I opened my browser's dev tools to inspect the element tree. I found the wrapper for the edit area which had a series of children. Each child was a wrapper for one line of text in the editor. I used the dev tools to copy the line wrapper and its descendants. Then I pasted a copy of it somewhere else among its siblings. I closed the dev tools and hit the save button.

Ok, thank you, I'll ask around if someone has any idea on how to do this. Btw, to be clear I was experimenting on the "source editor", meaning the editor used when you select "source editor" in your preferences.

Thank you again.

Ah. So the 2010 editor then. Thanks for the clarification.