If I was designing Syntax Highlighter I would include the copy code button but make it available to possibly members (or certain groups only) and if its possible prevent anyone not a member of the group to not even highlight and copy code.  Would help force people to join and become a member if they want the code.

Example via CSS class ".logged-in"

Pre Code display:none; for Guest

but "display: none;" can be deactivated in the CSS by the guests

How can you highlight HTML/XML code with this plugin?

As soon as I try to post HTML content as "code", wpForo remove all tags, or just display the HTML tags (even worse).

Same for XML or Handlebars.

Example:

Attached file: HTML in Visual Code

Without "code":

<li class="sfx flexrow sfx-line">
<div class="sfx-data">
<input type="text" name="sfxLine" value="" placeholder="d20, d6, df..." />
</div>
<div class="sfx-data">
<input type="text" name="sfxLine" value="" placeholder="1, 20, 6..." />
</div>
<div class="sfx-data sfx-double">
<select name="sfxLine">
{{selectOptions specialEffectsMode localize=true}}
</select>
</div>
<div class="sfx-controls">
<a class="sfx-control sfx-delete" title='{{localize "DICESONICE.Delete"}}'><i class="fas fa-trash"></i></a>
</div>
</li>

With "code":

<li class="sfx flexrow sfx-line">
<div class="sfx-data">
<input type="text" name="sfxLine" value="" placeholder="d20, d6, df..." />
</div>
<div class="sfx-data">
<input type="text" name="sfxLine" value="" placeholder="1, 20, 6..." />
</div>
<div class="sfx-data sfx-double">
<select name="sfxLine">
{{selectOptions specialEffectsMode localize=true}}
</select>
</div>
<div class="sfx-controls">
<a class="sfx-control sfx-delete" title='{{localize "DICESONICE.Delete"}}'><i class="fas fa-trash"></i></a>
</div>
</li>