Please see here on what is possible and what is not with the "team" field in jira
You can make the field optional from the field configuration screen.
You can select values from the team field if you have added teams in the porfolio. If there are no teams in porfolio then you won't be able to add new teams from JIRA User interface. You have to add the teams from within portfolio.
Use the above link, if it doesn't work, remove everything after "?" in the link generated.
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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