Add license key setting field

wp-crm-system.php

<?php
// Add license key settings field
function wpcrm_custom_license_field() {
  //get current tab
	global $wpcrm_active_tab;
	if ($wpcrm_active_tab == 'licenses') {
		//your license key field
	}
}
add_action( 'wpcrm_system_license_key_field', 'wpcrm_custom_license_field' );

/*Example license key field from one of our plugins
/*Your plugin will need to handle saving the license key, license status, as well as checks for valid licenses on your website
------------------------------------------------------------------------------------------------------------------------------*/

$wpcrm_my_plugin_key = get_option( 'wpcrm_my_plugin_license_key' );
$wpcrm_my_plugin_status = get_option( 'wpcrm_my_plugin_license_status' );
?>

<tr valign="top">
	<th scope="row" valign="top">
		<?php _e('My Custom License Key','wp-crm-system'); ?>
	</th>
	<td>
		<input id="wpcrm_license_key" name="wpcrm_license_key" type="text" class="regular-text" value="<?php esc_attr_e( $wpcrm_my_plugin_key ); ?>" />
		<label class="description" for="wpcrm_license_key"><?php _e('Enter your license key','wp-crm-system'); ?></label>
	</td>
</tr>
<?php if( false !== $wpcrm_my_plugin_key ) { ?>
	<tr valign="top">
		<th scope="row" valign="top">
			<?php _e('Activate License','wp-crm-system'); ?>
		</th>
		<td>
			<?php if( $wpcrm_my_plugin_status !== false && $wpcrm_my_plugin_status == 'valid' ) { ?>
				<span style="color:green;"><?php _e('active'); ?></span>
				<?php wp_nonce_field( 'wpcrm_plugin_license_nonce', 'wpcrm_plugin_license_nonce' ); ?>
				<input type="submit" class="button-secondary" name="wpcrm_my_plugin_deactivate" value="<?php _e('Deactivate License','wp-crm-system'); ?>"/>
			<?php } else {
				wp_nonce_field( 'wpcrm_plugin_license_nonce', 'wpcrm_plugin_license_nonce' ); ?>
				<input type="submit" class="button-secondary" name="wpcrm_my_plugin_activate" value="<?php _e('Activate License','wp-crm-system'); ?>"/>
			<?php } ?>
		</td>
	</tr>
<?php } ?>