Isn't this a one line change to the snippet?
Add the line ...
placeholder='{tag || null}'
... inside the input element like so (approx line 68) ...
<input
...
maxlength='{maxlength || null}'
placeholder='{tag || null}'
readonly='{isModifiable ? null : ""}'
...
</input>
(the attributes are coded in alphabetical order hence why I've inserted between maxlength and readonly attributes) .
Use comment instead of tag if you want to use the comment attribute. My first attempt was to use title e.g.
EDIT f01 = formonly.field, TITLE="Label Text";
... but unfortunately it doesn't look like the title attribute is passed into the formfield template (its used in table column)
Reuben