I do not believe Mimestream has support for direct integration like some other mail apps. Therefore, the mailto action linked above that works with any mail client app as long as it is setup as your default mail app is your best options. After you install it, you can rename it “Send to Mimestream” if that is easier for you to remember what it’s doing.
You might get some value from looking over the Mail integration guide as well to understand some of the sending options - like you might find it better to send using Apple Mail or Markdown mail options even if you are not using Apple Mail as your mail client, but want to be able to use Markdown to compose rich text.
Hope this helps!