<?php /* Template Name: Dummy Practice Page*/?>
<div id="main-content" class="main-content">
    <div class="main-content-inner">
            <form method="post">
            <p><div>
                <input name="nametxt" id="nametxt" type="text" style="height:30px; width: 350px; " maxlength="5" placeholder="Name" required><br>
            </p></div>
            <p><div>    
                <input name="designationtxt" id="designationtxt" type="text" style="height:30px; width: 350px; " maxlength="50" placeholder="Designation" required><br>
            </p></div>
            <p><div>
                <input name="designationtxt" id="descriptiontxt" type="text" style="height:30px; width: 350px; " maxlength="1000" placeholder="Description" required><br>
            </p></div>

                <input id="submitbtn" type='submit' style="height:40px; width: 130px; padding:10px; color:dodgerblue; background-color:black; border-radius:20px;   " name='Submit' value='Add Member' /><br><br>

            </form>
    </div>
</div>
<?php
     global $wpdb;
    $table = $wpdb->prefix.'tboxteam';
     $data = array('name' =>  isset($_POST['nametxt']), 'designation' =>  isset($_POST['designationtxt']), 'description' =>  isset($_POST['descriptiontxt']));
     $format = array('%s','%s','%s');
     $wpdb->insert($table,$data,$format);
?>

<?php /* Template Name: Dummy Practice Page*/?>
<div id="main-content" class="main-content">
    <div class="main-content-inner">
            <form method="post">
            <p><div>
                <input name="nametxt" id="nametxt" type="text" style="height:30px; width: 350px; " maxlength="5" placeholder="Name" required><br>
            </p></div>
            <p><div>    
                <input name="designationtxt" id="designationtxt" type="text" style="height:30px; width: 350px; " maxlength="50" placeholder="Designation" required><br>
            </p></div>
            <p><div>
                <input name="designationtxt" id="descriptiontxt" type="text" style="height:30px; width: 350px; " maxlength="1000" placeholder="Description" required><br>
            </p></div>

                <input id="submitbtn" type='submit' style="height:40px; width: 130px; padding:10px; color:dodgerblue; background-color:black; border-radius:20px;   " name='Submit' value='Add Member' /><br><br>

            </form>
    </div>
</div>
<?php
     global $wpdb;
    $table = $wpdb->prefix.'tboxteam';
     $data = array('name' =>  isset($_POST['nametxt']), 'designation' =>  isset($_POST['designationtxt']), 'description' =>  isset($_POST['descriptiontxt']));
     $format = array('%s','%s','%s');
     $wpdb->insert($table,$data,$format);
?>

• database
• forms

2 Answers 2

<?php /* Template Name: Dummy Practice Page*/?>
<div id="main-content" class="main-content">
  <div class="main-content-inner">
        <form method="post">
        <p><div>
            <input name="nametxt" id="nametxt" type="text" style="height:30px; width: 350px; " maxlength="5" placeholder="Name" required><br>
        </p></div>
        <p><div>    
            <input name="designationtxt" id="designationtxt" type="text" style="height:30px; width: 350px; " maxlength="50" placeholder="Designation" required><br>
        </p></div>
        <p><div>
            <input name="descriptiontxt" id="descriptiontxt" type="text" style="height:30px; width: 350px; " maxlength="1000" placeholder="Description" required><br>
        </p></div>

            <input id="submitbtn" type='submit' style="height:40px; width: 130px; padding:10px; color:dodgerblue; background-color:black; border-radius:20px;   " name='Submit' value='Add Member' /><br><br>

        </form>
    </div>
 </div>
 <?php
 // Checking if form submitted 
 if(isset($_POST['Submit'])){

       // Fetching values if they are not empty.
    if( isset($_POST['nametxt']) || isset($_POST['designationtxt']) ||isset($_POST['descriptiontxt']) ){

        global $wpdb;

        $data = array('name' =>  $_POST['nametxt'], 'designation' =>  $_POST['designationtxt'], 'description' =>  $_POST['descriptiontxt'] );


        $table = $wpdb->prefix.'tboxteam';

        $format = array('%s','%s','%s');

        $wpdb->insert($table,$data,$format);
    }
 }

 ?>

PHP isset returns boolean value to check if the array index exists. https://www.php/manual/en/function.isset.php

"name" => isset($_POST["nametxt"]) ? $_POST["nametxt"] : ""

This is what you should do