Format Subject and Message from HTML form to mailto

I currently have an HTML form that is supposed to open up the mail app on windows and I want the input and textarea to autofill the subject and message area of the email if possible.

<form action="mailto:RECIEVER_EMAIL@EMAIL.COM"
 method="POST"
 enctype="multipart/form-data"
 name="EmailForm">
        <input type="text" size="19" name="Contact-Name" maxlength="100" value="EMAIL SUBJECT">

       <textarea name="Contact-Message" cols="50" rows="5" maxlength="1000">EMAIL MESSAGE</textarea> 
       <button id="submit-btn" type="submit" value="Submit">Submit</button></form>

CURRENT OUTPUT

I was wondering if it is possible to autofill the subject area of the email with the value in the input tag and have the message area autofilled with the text area value without the "Contact-Message=" method and the "+" between spaces. I am hosting this on github pages so I, unfortunately, can't use php.