The creation of a new collection will only be done if the search is for multiple domains. If you are only searching one domain, you should be using the sitesearch parameter not a collection.
Please note: Departmental websites are an exception to this rule.
For a new collection, submit a request to SNSMR. Include the following information:
Using the Site Search Parameter
What is a collection?
What is my collection?
How do I add a site to a collection?
How often is the collection updated?