AFAIK this is not possible out of the box but should be fairly easy to accomplish with a small javascript snippet.
//find the datagrid
var dataGrid = document.querySelector('.mx-datagrid.mx-name-products');
//find the select element and search button
var itemsSelect = dataGrid.querySelector('.mx-grid-search-input.mx-name-itemsField > select')
searchButton = dataGrid.querySelector('.mx-grid-search-controls > button.mx-grid-search-button')
//whenver the select option is changed trigger a click on the search
itemsSelect.onchange=function() {
searchButton.click();
};
Make sure to put the javascript snippet after the datagrid in the modeler so that it loads after the grid is loaded.