In a form editiong we can add a custom button with action
This is done in edit/add options property buttons
<?php 
require_once '../../../../php/demo/tabs.php';
?>
<!DOCTYPE html>
<html>
  <head>
    <title>jqGrid PHP Demo</title>
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/jquery-ui.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/trirand/ui.jqgrid.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/ui.multiselect.css" />
    <style type="text">
        html, body {
        margin: 0;            /* Remove body margin/padding */
        padding: 0;
        overflow: hidden;    /* Remove scroll bars on browser window */
        font-size: 75%;
        }
        
    </style>
    <script src="../../../../js/jquery.min.js" type="text/javascript"></script>
    <script src="../../../../js/trirand/i18n/grid.locale-en.js" type="text/javascript"></script>
         <script src="../../../../js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script> <script type="text/javascript">         
    $.jgrid.no_legacy_api = true;
    $.jgrid.useJSON = true;
    $.jgrid.defaults.width = "700";
    </script>
     
    <script src="../../../../js/jquery-ui.min.js" type="text/javascript"></script>
  </head>
  <body>
      <div>
          <?php include ("grid.php");?>
      </div>
      <br/>
      <?php tabs(array("grid.php"));?>
   </body>
</html>
grid.php.
<?php
require_once '../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/PHPSuito/jqGrid.php";
// include the driver class
require_once ABSPATH."php/PHPSuito/DBdrivers/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES 'utf8'");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand = 'SELECT OrderID, CustomerID, Freight, OrderDate, ShipCity FROM orders';
// set the ouput format to json
$grid->table = 'orders';
$grid->dataType = 'json';
$grid->setPrimaryKeyId('OrderID');
$grid->serialKey = true;
// this enables returning the result after insert in case of serialKey
$grid->getLastInsert = true;
// Let the grid create the model
$grid->setColModel();
// Set the url from where we obtain the data
$grid->setUrl('grid.php');
// Set some grid options
$grid->setGridOptions(array(
    "rowNum"=>10,
    "rowList"=>array(10,20,30), 
    "sortname"=>"OrderID",
    "height"=>150
));
// Change some property of the field(s)
$grid->setColProperty("OrderDate", array(
    "formatter"=>"date",
    "formatoptions"=>array("srcformat"=>"Y-m-d H:i:s","newformat"=>"m/d/Y"),
    "search"=>false,
    )
);

$grid->setUserTime('m/d/Y');
$grid->setUserDate('m/d/Y');

$grid->setColProperty("OrderID",array("editable"=>false, "key"=>true));
$grid->setSelect('CustomerID', "SELECT CustomerID, CompanyName FROM customers");
// Enable toolbar searching
$grid->navigator = true;
$grid->setNavOptions('navigator', array("del"=>false,"excel"=>true,"search"=>false,"refresh"=>false));
$grid->setNavOptions('edit', array(
    "height"=>'auto',
    "dataheight"=>"auto", 
    "width"=>"auto", 
    "reloadAfterSubmit"=>false,
// definition of custom button in edit form
    "buttons" => array(
        array(
            'side' => "right",
            'text' => "Custom",
            'position' => "first",
            'click' => 'js:function( form, params, event) {
                alert("Custom action in edit form");
            }'
        )
    )
));
$grid->setNavOptions('add', array("height"=>'auto',"dataheight"=>"auto", "width"=>"auto", "reloadAfterSubmit"=>false));

$after = <<< AFTER
function( data, postdata, oper)
{
    // when grid -> getLastInsert = true;  the key is send like keyName#Value
    var retdata = data.responseText.split('#');
        //console.log(retdata);
        //console.log(postdata);
    if( retdata[0] && retdata[1] != null )
    {
        // make the selection
        setTimeout(function(){ $('#grid').jqGrid('setSelection', retdata[1]) }, 200);
        // return the key so that it will be set in grid
        return[true,'',retdata[1]];
    }
    return [false, 'Error during add'];
}
AFTER;

$grid->setNavEvent('add', 'afterSubmit', $after);
// Enjoy
$grid->renderGrid('#grid','#pager',true, null, null, true,true);
$conn = null;
?>