Hello, first you have to switch to the source editor and use the template you have already mentioned:

𝐇𝐞𝐥𝐥𝐨, {{Username}}

Well, first of all they have to install the script InputUsername and the corresponding template on their wiki.

Then, they can add it to their profile (but not their message wall for what I know).

It worked dw

Apparently the solution has already been implemented by the OP, but it was forgotten to confirm this here. 🤷‍♂️