How do I know what getFieldById of jira-bridge requires?

Hello,

I am trying to understand the turorial at https://developer.atlassian.com/platform/forge/manifest-reference/modules/jira-ui-modifications/

My question is about the getFieldById method used in the code example eg.:

const priority = getFieldById('priority');

How do I know what IDs are available for modification? Can I add some of my own?

Best regards