Pleased to meet you, I am working on a platform which has an element (select option) bootstrap that "inspect" shows me the item you want to change but I'm not sure that this document is.
When I look at the source, I only see the css and styles on the right but in the element shown it shows me what I want but not the place where I can find and edit it
In this case you must add some jquery and javascript code: in your hmtl of that page.
Assuming this is my selection to which I will add the new option:
<select name="datatable-buttons_length" class="form-control input-sm">
<option value="10">10</option>
<option value="20">20</option>
<option value="30">30</option>
</select>
my javascript and jquery code:
<script>
//nueva opcion para ser agregada
var option_new = $('<option value="1000">1000</option>');
//agregando option_new
$('select[name=datatable-buttons_length').append(option_new);
</script>
in your case the select does not have id, for which I did it x the name, you can do it x id, name, class css..etc ..:
I hope it serves you ..suerte