How to use JWT in unity?

It's more of a server question than Unity. You will send a request to the server with for example the users mail (no JWT here). The server will then generate the JWT which you receive and save in Unity. In every other request you will send the JWT too and the server verifies that its valid.

An example PHP script:

 <?php 
 use \Firebase\JWT\JWT; //installed via Composer
 
 function Get($mail){
     $token = array(
         'iss' => 'YourName',
         'exp' => time() + 3600,    //valid for 1h
         'iat' => time(),
         'eml' => $mail,    //own value
     );
     return JWT::encode($token, JWTKey); //they secret key only your server knows
 }
 
 function Verify($token, $mail){
     try{
         $userDecoded = JWT::decode($token, JWTKey, array('HS256'));
         if(!isset($userDecoded->eml) ||
           strcmp($userDecoded->eml,$mail) != 0){
             http_response_code(417);
             exit;
         }else{
             return true;
         }
     }catch(\Exception $e){
         echo 'Caught exception: ',  $e->getMessage(), "\n";
         http_response_code(417);
         exit;
     }
 }
 ?>

Best,
Thomas