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In your Jira Cloud instance, select the Settings cog, and then select Issues.
Under Fields in the Issues left sidebar menu, select Custom Fields, then select Create custom field.
Select the type of field you want to create, then select Next. Atlassian has a list of available custom field types.
Add a name and a description (optional) for your field, then select Create.
Add your new custom field to the screens of the projects where you want it to appear by selecting the corresponding checkboxes, and then select Update.
Your custom fields are now available for selection when needed when configuring your post functions.