Just to finalise this for those interested. I followed gmayor's advice (thanks gmayor) in relation to:

ActiveWindow.EnvelopeVisible = True

I created the above macro. Then to insert I used the field function (Insert -->Quick Parts --> Field

In field names select Macrobutton then select the macro you created (with some display text) click ok. Double click on the text and the drop down appears.

FYI I only need the drop down to appear as different users will be sending the email to different email recipients dependant on the intial email sent to them.

I appreciate your assistance