The js_sign plugin changes the "Sign" button on the compose page to use an ajax
callback to get the selected profiles signature and javascript to insert the sig
into the message at the current cursor position (the default Hastymail2 behavior
is to append the signature at the bottom of the message). To enable this plugin
add or uncomment the following in your hastymail2.conf file:

plugin = js_sign

Rebuild the hastymail2.rc file and logout then back in to enable the plugin.
This plugin is a part of Hastymail so questions and comments can be directed to
our forums, mailing lists, or directly to me at jason@hastymail.org.
