Hi @hikmahaz,
{{erpnext.url}}/api/resource/Agency?fields=["category"]&filters=[["category","=","f9c3093c05"]]
only category id display only.
Otherwise, first, fetch category_name and priority from Category in a custom field in Agency Doctype then API set like
{{erpnext.url}}/api/resource/Agency?fields=["category_name"]&filters=[["category_name","=","Test1"]]
Thanks.