// Base64 encode it and replace all + with -, replace all / with _,
// and remove the trailing = to make it URL-safe:
btoa(
  "From: [email protected]\r\n" +
  "To: [email protected]\r\n" +
  "Subject: Subject Text\r\n\r\n" +

  "The message text goes here"
).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');

// "RnJvbTogc3Rhbi50b2xlc0B5YWhvby5jb20NClRvOiBzdGFuLnRvbGVzQHlhaG9vLmNvbQ0KU3ViamVjdDogU3ViamVjdCBUZXh0DQoNClRoZSBtZXNzYWdlIHRleHQgZ29lcyBoZXJl"