Custom Form within WordPress Admin Dashboard Custom Menu Page

plugin.php

<?php


//Check for hidden field value and select appropriate API field needed
          
          if( $_POST['action'] == 'filterdates' ){
            check_admin_referer( 'filter-date', '_wpnonce_filter-date' );                       
            $requestUrl = $sDomain ."/api/v3/events?\$expand=instances";  
            
            $currentYear = date("Y");

            switch( $_POST['month'] ){
                case 'January':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=1/1/" . $currentYear . "&instanceStart_to=1/31/". $currentYear;
                break;

                case 'February':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=2/1/" . $currentYear . "&instanceStart_to=2/28/". $currentYear;
                break;

                case 'March':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=3/1/" . $currentYear . "&instanceStart_to=3/31/". $currentYear;
                break;

                case 'April':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=4/1/" . $currentYear . "&instanceStart_to=4/30/". $currentYear;
                break;
                
                case 'May':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=5/1/" . $currentYear . "&instanceStart_to=5/31/". $currentYear;
                break;

                case 'June':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=6/1/" . $currentYear . "&instanceStart_to=6/30/". $currentYear;
                break;

                case 'July':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=7/1/" . $currentYear . "&instanceStart_to=7/31/". $currentYear;
                break;

                case 'August':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=8/1/" . $currentYear . "&instanceStart_to=8/31/". $currentYear;
                break;
            
                case 'September':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=9/1/" . $currentYear . "&instanceStart_to=9/30/". $currentYear;
                break;

                case 'October':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=10/1/" . $currentYear . "&instanceStart_to=10/31/". $currentYear;
                break;

                case 'November':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=11/1/" . $currentYear . "&instanceStart_to=11/30/". $currentYear;
                break;

                case 'December':
                    $requestUrl = $sDomain ."/api/v3/events?\$expand=instances&instanceStart_from=12/1/" . $currentYear . "&instanceStart_to=12/31/". $currentYear;
                break;
            
            }

        }else{
                       
            $requestUrl = $sDomain ."/api/v3/events?\$expand=instances";
        }

    ?>
    
    <!-- form for filtering view -->
    
    <form id="dates-filter" method="POST" action="">
        <div class="tablenav top">
            <div class="alignleft actions bulkactions">
                <select name="month" id="month">
                    <option value="">Choose a Month</option>
                    <option value="January">January</option>
                    <option value="February">February</option>
                    <option value="March">March</option>
                    <option value="April">April</option>
                    <option value="May">May</option>
                    <option value="June">June</option>
                    <option value="July">July</option>
                    <option value="August">August</option>
                    <option value="September">September</option>
                    <option value="October">October</option>
                    <option value="November">November</option>
                    <option value="December">December</option>
                </select>
                <?php wp_nonce_field( 'filter-date', '_wpnonce_filter-date' ) ?>
                <input name="action" type="hidden" value="filterdates" />

                <input type="submit" id="filterDate" class="button action" value="Filter Dates">
            </div>
        </div>