Full instructions are here: http://kb.mozillazine.org/Command_line_arguments_-_Thunderbird

Thanks for your reply but I found the info in the link you sent difficult to understand or implement. I'm trying to edit a "Send to" link via shell:sendto by editing the Mail link properties - see attached Mail Properties.jpg. This partially works - attached Mail Result.jpg - but notice that while the Thunderbird Write window shows the attachment C:\Users\"username"\desktop\hosts at bottom, above that line it shows 0 bytes.

So it seems I've got the solution partially in place but am somehow missing an argument in the properties so I'm not really attaching the hosts file...

The link I gave had the parameters you wanted to include a default subject and body to the shell sendto shortcut. Have you tried sending what you have in compose window, despite it showing zero length?

Thx again! I did try to send the note and obtained the following message - see attached "Failed to attach.jpg"

This all started with my desire to change the default subject and body text that I had by default -- and whatever the default properties were (whatever came standard with MS Windows 10, which, of course I lost or deleted) I was always able to right click the intended attachment, bring up the TB compose window to enter the address with the attachment clearly showing , etc. and successfully send it on it's way.

Anyway, not sure what rights I should have to my desktop or other files that I didn't have before but,

The error in sending may be a setup in WIndows to adjust. I have no problem sending jpg that way, and I also see the zero bytes statement.

Interesting... I'd have no clue where Windows would address this.

What started me on this is that the default Windows 'Mail recipient' link/shortcut results in (see attached "Default WIndows Write screen.jpg"} the default Subject and body text as shown in the attachment. This is what obtains when the Mail recipient shortcut consists of an empty file "Mail recipient" with the extension MAPIMail. Creating that empty file and using it as the send-to shortcut works fine if you don't mind the message.

Anyway, none of this is critical - simply looking to customize that message but not motivated to go much further. The default message is in a Wndows dll file, sendmil.dll which I haven't a clue (or inclination) how to edit -- it's hex...