Make a Custom Field of the Type: "Message Custom Field (for view)" or "Message Custom Field (for edit)" - these are from the https://ecosystem.atlassian.net/wiki/display/JTOOL/JIRA+Toolkit+Plugin

Add the Default Value:

<div class="field-group aui-field-wikiedit comment-input">
<a href="https://myserver.mydomain.dk/display/SITE/A+Page" target="_blank" class="aui-button aui-button-primary aui-style">Click to see a Page</a>
</div>

or for an Alert

<div class="field-group aui-field-wikiedit comment-input" border>
<a class="aui-button aui-button-primary aui-style" onclick="alert('Access Mangement = Access changing issues, eg: Firewall, LDAP, Kerberos, Account add/delete/change and similar.\n\nCapacity Management = Capacity related issues, eg: Disk/SAN/Memory/HW Changes, Expansions, Measurements and similar')">Click here for ITIL Tag explanation</a>
</div>

This will render a button