I want to enable a custom URI protocol in my issue descriptions so that a link of the type
<a href="myapp:args">click here</a> will work.
For this I need to enable HTML editing (although the description will be posted by the REST API)
I found this page
https://confluence.atlassian.com/jirakb/how-to-enable-html-in-jira-text-fields-771895067.html
And followed the instructions, but when I get to the Wike Renderer Macros Plugin, there's no "Manage Plugin" option. I can exappand the list of 7 out o7 modules, but they don't include "Html", and in each case hovering over the module shows the message "Module cannot be modified".
I'm fairly new to Jira, so I'm hoping I've missed something really basic (e.g. putting myself in an edit mode)