This. I tried multiple ways yesterday, there is no way to change the "value" attribute of the textarea simply by editing the address.

However, if you're certain you saw somebody do this on EUSCL, try finding the post and recreating the link. Not sure what else we can tell ya here.

Can’t change it because there’s no value element to change

Textareas work differently than other input fields, they do not take a value attribute (like roadtree already said). You have to place the textarea value between the textarea tags.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea

When you're submitting form (sending pm) with get method it appends form data into the url in name & value pairs which you can edit. Like you can set in your link the user you want to send pm to (u=123456) or subject of the pm (subj=something). As textarea has no value attribute you can't simply set it (like '&Post=whatever').

tl//dr - Not possible afaik.