There is a Welcome Message text box as part of the Arlo Enrolment option. You can choose to use the default message, or override it with a custom message. The message can contain various types of Text including Moodle auto text, HTML tags etc, although I expect you already know this.
Given the box can contain HTML, the message above was taken from an HTML email from the Arlo platform in order to mimic the look and feel. I then edited this as necessary in a third party HTML editor (as there a LOADS of nested tables), but this has the side effect of using HTML encoding for the greater than sign (i.e. `>`), which was encoded to `>`
In fact, you can simply try this yourself in your reply, type `>` then switch to advanced view, and it will turn into a greater than sign. However, when I figured this out, I simply re-added the actual greater than sign in the Welcome Message text box, which then turns into a mini arrow, and all is fine.
The “firstname” placeholder I just could not get to work, although “fullname” will do for now.