So,
1. create the custom field
2. add it to the screens used by the project
It can get a little more complicated if your screen schemes and field configs are shared between projects and the other project don't want these additional fields
Here are the guides from atlassian:
https://confluence.atlassian.com/adminjiraserver/adding-a-custom-field-938847222.html
https://confluence.atlassian.com/adminjiraserver/project-screens-schemes-and-fields-938847220.html
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