add as you would any custom field:
https://confluence.atlassian.com/adminjiraserver/adding-a-custom-field-938847222.html
Select the standard text field (multi-line is my usual choice)
Once the field is setup, go to the field configuration and choose 'wiki-style renderer'
https://confluence.atlassian.com/adminjiracloud/issue-field-configurations-844500794.html
I think that's what you are asking for...
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