I finally solve the problem by removing
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
and adding
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
to Web.config
Now I can call the service using
http://localhost:2586/Service1.asm/HelloWorld?parameter1=abc¶meter2=cde