How can I pass a variable to wp_ajax action?

You’d need to put the variable’s value onto the page so that javascript can pass it back to PHP using ajax. Ajax functions are a completely separate “page load” so even if you made it global it won’t work. The best way to do this is using “wp_localize_script” (https://codex.wordpress.org/Function_Reference/wp_localize_script)

Here’s an full example of how you could do it:

//First enqueue your javascript in WordPress
    function your_prefix_enqueue_scripts(){

        //Enqueue your Javascript (this assumes your javascript file is located in your plugin in an "includes/js" directory)
        wp_enqueue_script( 'your_unique_js_name', plugins_url('js/yourjavascriptfile.js', dirname(__FILE__) ), array( 'jquery' ) );

        //OR (simpler but not recommended)  
        wp_enqueue_script( 'your_unique_js_name', 'http://domain.com/myjavascriptfile.js', array( 'jquery' ) );

        //Here we create a javascript object variable called "youruniquejs_vars". We can access any variable in the array using youruniquejs_vars.name_of_sub_variable
        wp_localize_script( 'your_unique_js_name', 'youruniquejs_vars', 
            array(
                //To use this variable in javascript use "youruniquejs_vars.ajaxurl"
                'ajaxurl' => admin_url( 'admin-ajax.php' ),
                //To use this variable in javascript use "youruniquejs_vars.the_issue_key"
                'the_issue_key' => $the_issue_key,

            ) 
        );  

    }
    add_action( 'wp_enqueue_scripts', 'your_prefix_enqueue_scripts' );

    //This is your Ajax callback function
    function your_ajax_callback_function_name(){

        //Get the post data 
        $the_issue_key = $_POST["the_issue_key"];

        //Do your stuff here

        //Create the array we send back to javascript here
        $array_we_send_back = array( 'test' => "Test" );

        //Make sure to json encode the output because that's what it is expecting
        echo json_encode( $array_we_send_back );

        //Make sure you die when finished doing ajax output.
        die(); 

    }
    add_action( 'wp_ajax_' . 'your_ajax_callback_function_name', 'your_ajax_callback_function_name' );
    add_action( 'wp_ajax_nopriv_' . 'your_ajax_callback_function_name', 'your_ajax_callback_function_name' );

And then in your javascript file do something like this:

jQuery(document).ready(function($){

    /**
     * When your ajax trigger is clicked
     *
     */
    $( document ).on( 'click', '.my-button', function(event){

        event.preventDefault();

        // Use ajax to do something...
        var postData = {
            action: 'your_ajax_callback_function_name',
            the_issue_key: youruniquejs_vars.the_issue_ke,
        }

        //Ajax load more posts
        $.ajax({
            type: "POST",
            data: postData,
            dataType:"json",
            url: youruniquejs_vars.ajaxurl,
            //This fires when the ajax 'comes back' and it is valid json
            success: function (response) {

                alert( response.test );

            }
            //This fires when the ajax 'comes back' and it isn't valid json
        }).fail(function (data) {
            console.log(data);
        }); 

    });

});