I think there might be some context missing from this question, but I'll take a stab at an answer based on a guess at what you're doing.
As an administrator, add the custom fields to the "screen" that is used for "create issue" in the project. Then you'll be able to add data to the issue as you create it. Later, if you want to edit them, add the fields to the "edit" screen. (Note that these may well be the same Jira "screen", so you may only have to do it once)
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