Put a variable in to email sent out

This is I am sure simple, but I can't seem to figure out how to insert a variable in to an email text. I am using an esp8226 board and sending email with smtp. I am having no issues sending text within the email for example:

  String htmlMsg = "<div style=\"color:#2f4468;\"><h1>Did you get this email?  The time is 4:54</h1><p>- Sent from ESP board</p></div>";
  message.html.content = htmlMsg.c_str();
  message.html.content = htmlMsg.c_str();
  message.text.charSet = "us-ascii";
  message.html.transfer_encoding = Content_Transfer_Encoding::enc_7bit;

But I can't figure out how to put a variable that I have within the code in to the email that is sent out. I am able to put a variable on to a web server by using the client.println (variable); . How do I do this in an email sent out?

I can post the entire code if necessary, but I figure it is something simple....

Here is the email function I am using:

void Testemail1()
{
  smtp.debug(1);

  smtp.callback(smtpCallback);

  ESP_Mail_Session session;

  session.server.host_name = SMTP_HOST;
  session.server.port = SMTP_PORT;
  session.login.email = AUTHOR_EMAIL;
  session.login.password = AUTHOR_PASSWORD;
  session.login.user_domain = "";

  SMTP_Message message;

  message.sender.name = "Dehumidifier Message";
  message.sender.email = AUTHOR_EMAIL;
  message.subject = "Email Test";
  message.addRecipient("", RECIPIENT_EMAIL);
  String htmlMsg = "<div style=\"color:#2f4468;\"><h1>Did you get this email?  The time is 4:54</h1><p>- Sent from ESP board</p></div>";
  message.html.content = htmlMsg.c_str();
  message.html.content = htmlMsg.c_str();
  message.text.charSet = "us-ascii";
  message.html.transfer_encoding = Content_Transfer_Encoding::enc_7bit;

  if (!smtp.connect(&session))
    return;

  if (!MailClient.sendMail(&smtp, &message))
    Serial.println("Error sending Email, " + smtp.errorReason());
  TestEmail = true;
}

Thanks!

String htmlMsg = "Did you get this email?\n The variable is ";
htmlMsg += String( variable );
htmlMsg += " units.  Have a nice day!";
...

Awesome! Thanks so much!

You have a nice day as well!