Q:

How to send dynamically value from javascript to hiddenfield

I have a variable "place", which is changing dynamically (get diffrent values for some actions). Here JS code.
google.maps.event.addListener(autocomplete, 'place_changed', function () {
  var place = autocomplete.getPlace();
});

How to get this variable to asp.net(C#) in code behind? 

A:

document.getElementById('myHiddenField').value = place;

Just keep in mind that client IDs are dynamic in ASP.NET, unless you're using ASP.NET 4 and set ClientIDMode = "Static" on that hidden field.  So you may need to add something like this to your Page_Load:
Page.RegisterStartupScript(this, "hiddenFieldIdSetter", "var hiddenFieldID = '"+ myHiddenField.ID +"';", true);

and then use the following javascript, instead:
document.getElementById(hiddenFieldID).value = place;

