To open a new email message in the user’s email client, I have
\useURL
[xxx]
[mailto:xxx@yyy.org]
[]
[{xxx}]

\setupinteraction[state=start,color=blue]

\starttext

send email to \from[xxx]

\stoptext

This sets the address to which the message is to be sent. Is there a way to set the subject of this message as well?

Alan