Invoke from the main menu

To create a custom menu item that invokes a search, use the MenuItem attribute. The following example creates a menu item that searches the Asset StoreA growing library of free and commercial assets created by Unity and members of the community. Offers a wide variety of assets, from textures, models and animations to whole project examples, tutorials and Editor extensions. More info
See in Glossary for free assets:

[MenuItem("Search/Free Asset Gifts")]
static void SearchStore()
{
    var storeContext = SearchService.CreateContext("store", "price=0");
    var viewState = new SearchViewState(storeContext,
        UnityEngine.Search.SearchViewFlags.DisableNoResultTips |
        UnityEngine.Search.SearchViewFlags.GridView);
    viewState.windowTitle = new GUIContent("Free Stuff for Christmas");
    viewState.queryBuilderEnabled = true;
    SearchService.ShowWindow(viewState);
}