the search from the Portal works client-side, and searches only by columns displayed on the view
you can add Metadata Filters to search on additional fields, but this would be represented as different fields to be search by: https://docs.microsoft.com/en-us/powerapps/maker/portals/configure/entity-lists#entity-list-filter-configuration
another option you can do is adding the field you want to make the search to the view, and hide it via JS
the below JS can be placed in your Entity List custom JavaScript section, the number 4 below represents the index of the column, so just change that to the index of the column you want to hide and search
$(document).ready(function () {
var list = $(".entitylist.entity-grid").eq(0);
list.on("loaded", function () {
// hide column by index
list.find("table thead th").eq(4).hide();
list.find("table tbody tr").each(function () { $(this).find("td").eq(4).hide(); });
});
});
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