The official Email PopClip Extensions is to copy the selected text to the content of the email, can you copy the selected text to the title of the email.
My idea is: send the selected text to omnifocus and my usual email address by email, so that I can manage my to-do items in both the mac system and the windows system, the mac system uses omnifocus to manage to-do items, in The Windows system uses Outlook software to manage emails, and emails are used as to-do items.