JavaScripr filter for input options that can filter by any word/symbol/letter, not by only first words and symbols of option value

I have a code:

<form>
<p>Filter by category<br />
<input type="text" id="searchz" name="search" value="-- " style="margin: 10px;width: 165px;" onkeyup="filter(this.value)">
<button type="button" onclick="ClearFields();">Clear search</button>
</form>

When call a function:

<?php echo $lists['categories'];?>

That function displays code like:

<select id="category_id">
<option>category_name</option> 
<option>-- subcategory_name</option>
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option> 
<option>-- subcategory_name</option>
</select>

And i have script that works:

<script>
function filter(keyword) {
    var select = document.getElementById("category_id");
    for (var i = 0; i < select.length; i++) {
        var txt = select.options[i].text;
        var include = txt.toLowerCase().startsWith(keyword.toLowerCase());
        select.options[i].style.display = include ? 'list-item':'none';
    }
}
function ClearFields() {

     document.getElementById("searchz").value = "";
}
</script>

But script filtering only for first letters and symbols, and for find subcategory i need to type in filter input "-- subcategory name" Need filtering by any word or letter/symbol of option value, thanks.