HTML Signature - Windows Client

Hello all,

HAve come to modernise a clients signature in GW.  I seem to remember that in tools/options/signatures there was an HTML button to edit the HTML of the signatures but in 18.3 I cannot find that button.  

Am I remembering correctly and if so has the button moved?  I can paste a graphical signature in and it looks OK but without being able to paste the actual code the signature is not responsive when viewed on mobile devices or other small screens.