I assume you are actually trying to email all the music files in a Playlist to someone. That seems to be what the "right-click a Playlist > Send To > Email / Web > Mail Recipient" function does.
The first issue that I think will make this impossible is that most email systems limit the size of attachments to quite small sizes. Traditional email systems like Outlook limit attachments to 10MB. That isn't much, and would probably be less than even a small Playlist of MP3 files.
Gmail appears to have a 25MB attachment size limit... but it isn't a real 25MB, and even if it was, that isn't very big. See
https://www.gmass.co/blog/gmail-email-size-limit/ Note that if you exceed the size limit, Gmail uploads the files to Google Drive and inserts a link to the files into the email. That seems to be what you are trying to avoid.
If you still want to go ahead and email a Playlist full of files, and you are using Windows 10 (earlier versions have a similar requirement), you should just need to set your Default Email application in Windows Settings. Search for "Default Apps" in the Windows 10 start menu, then select the Gmail App as your default email application. If you are using the web interface for Gmail I'm not sure if you can use Send To command. I'll let you look into that if required.
Note that when using the Send To function for this, the email created seems to open in the background, and is therefore not visible. You will need to switch (Alt+tab) to the email window to complete the addressing and sending of the email.
If you wanted to just email a Playlist and not the music file, select the Playlist in MC, use the "File > Export Playlist" function to export the Playlists to a file (select the Playlist file format you want to use, such as M3U), save the file to an appropriate location, open your email program, create an email to send to whoever you want to have the Playlist, attach the Playlist file and send the email.
