Setting hidden input value in Javascript, then accessing it in codebehind

I have been trying to set the value of a hidden input by using Javascript and then access the value from within my C# codebehind. When I run the code that is copied below, the value that is assigned to assignedIDs is “”, which I assume is the default value for a hidden input. If I manually set the value in the html tag, then assignedIDs is set to that value.

ASP .NET MVC How to increase the width of an inputfield with @Html.EditorFor

I just cant fix something that seems so easy.
I want a textbox (editorfor) for a model property and I would like to increase its width but nothing is happening. I’m using the code as listed below. I tried setting the width to 500px but nothing happens. Ideally I would like the textbox to stretch over the full width of the container. Any ideas?