MB Creation

Popup a message on your WordPress blog is now very easy.
If you want to promote your brand new newsletter functionnality, display advertising on your homepage or get more Facebook fans you can use Simple Popup Manager to popup a message all over your content in a lovely “lightbox effect”. You can customize your message using a WP editor and of course specify the width and the height of the message box. You can limit the display of the message on either session or days basis.
<ul>
<li>Choose wether the popup will be displayed only on homepage or on every page</li>
<li>Set size, width and threshold of the popup (you can disable mobile)</li>
<li>Customize background-color and opacity</li>
<li>Popup content is just a normal WordPress editor (you can use shortcode)</li>
</ul>

Simple Popup Manager

Simple Popup Manager allows you to easily popup and display a simple message box on your WordPress front page.

Dreb Bitanghol

This field will provide you more options and flexibility on how you would like to present an instruction or message to your client’s (or site in general) admin interface.
Features:
<ul>
<li>Support PHP.</li>
<li>Option to hide label when field is displayed.</li>
</ul>
Compatible with Advanced Custom Fields:
<ul>
<li>4 (tested up to 4.4.6)</li>
<li>5 (PRO, tested up to 5.3.7)</li>
</ul>
PS: You need <a href="https://wordpress.org/plugins/advanced-custom-fields/" rel="ugc">Advanced Custom Fields</a> installed to use this.
PPS: Even more awesome if you have <a href="http://www.advancedcustomfields.com/pro/" rel="nofollow ugc">Advanced Custom Fields PRO</a> installed.
Important note: Use this with utmost care.

ACF Enhanced Message Field

Exactly what I needed

Exactly what I needed. You could improve the plugin by making the textarea field look more like an IDE, with system fonts, syntax highlights and so on.

Perfect

Awesome!!

Adds an enhanced version of the default Message field to accept PHP and certainly no wpauto().

Simple Popup Manager vs ACF Enhanced Message Field