call function from ***.aspx.cs from javascript function

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ASP.NET

Javascript

HTML

Hello,

I have the folowing html code :

<select name="myselect" id="myselect" >
    <option value="2">3 luni  - pret = 15$</option>
    <option value="4">6 luni  - pret = 30$</option>
    <option value="6">1 an    - pret = 60$</option>
    <option value="8">5 ani   - pret = 250$</option>
    <option value="10">PeViata - pret = 500$</option>
</select>

Each time when the item changes I want to call a function from my aspx.cs file.
I really don't know how to do this (this is my first app using asp.net) .
I thought that I could call a js function when the item is changed . below is my js function.

function selectorFunction() {
        var x = document.getElementById("myselect").value;
        <% selectPrice(Convert.ToInt32(x)); %>
}

The issue with this function is that var x does not exists in that context.
Can you please help me figure out how can I call a function from my aspx.cs file when a different item is selected.

Thank you in advance.