[laimas]

$(function() {
    $('input[name="idcopy"]').blur(function() {
        var v = this.value, 
            a = $('#link'), 
            p = a.prop('href'), 
            r = new RegExp('&'+this.name+'=.+'), 
            d = '&'+this.name+'='+v;
        if(v) {
            p = ~p.indexOf('&'+this.name) ? p.replace(r, d) : p + d   
        } else p = p.replace(r, '');
        
        a.prop('href', p);
    }).on('input', function() {
        this.value = this.value.replace(/\D/g, '')
    });
});

<a id="link" href="/reg/live/buyer.php?obj=1810&action=addform">GO</a>
<input name="idcopy" />