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>
This will render a button