Welcome to the Atlassian community! In order to add a custom field, you can follow the guides in the links below. Not sure if you use CLoud version or Server, so I included both. Let us know if you have any other questions.
Add a field on server version: https://confluence.atlassian.com/adminjiraserver/adding-a-custom-field-938847222.html
Add a field on cloud version: https://confluence.atlassian.com/adminjiracloud/adding-editing-and-deleting-a-custom-field-776636410.html
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