Like this:

<kbd>![RMB][RMB] RMB</kbd>

     <kbd>![RMB][RMB] RMB</kbd>
    
      [RMB]: https://i.sstatic.net/LPwD4.png (Right Mouse Button)

The mouse icon is simply an image, embedded via markdown syntax just like any other image.

I and some other users have [written a userscript][1] which allows these and other `<kbd>` buttons to be inserted with a single keyboard shortcut.

---

The links for each icon are as follows:

<kbd>![RMB][RMB] RMB</kbd> `https://i.sstatic.net/LPwD4.png`

<kbd>![LMB][LMB] LMB</kbd> `https://i.sstatic.net/FwrAW.png`

<kbd>![MMB][MMB] MMB</kbd> `https://i.sstatic.net/OASpJ.png`

<kbd>![MW][MW] MW</kbd> `https://i.sstatic.net/v1vyT.png`

The icons originate from the [wiki][2], but it was decided during the development of the userscript to host them on imgur in case the wiki versions changed.


[RMB]: https://i.sstatic.net/LPwD4.png (Right Mouse Button)



  [LMB]: https://i.sstatic.net/FwrAW.png (Left Mouse Button)

  [MMB]: https://i.sstatic.net/OASpJ.png (Middle Mouse Button)

  [MW]: https://i.sstatic.net/v1vyT.png (Mouse Wheel)


  [1]: https://blender.meta.stackexchange.com/a/391/599
  [2]: https://wiki.blender.org/index.php/Meta:Guides/Templates#Mouse_hotkeys