You will need to handle javascript onchange event for the ddl. Detect in the
event when the desired item is selected and call myForm.submit(). This will
initiate a postback.

Well,

I really don't know how to do this.

In my User Control Page_Load I know I have to do someting like that
ddlTypeTransaction.Attributes.Add("onchange", "??");

?? don't know exactly what do here

On my Default page I have this too
       HtmlGenericControl Include = new HtmlGenericControl("script");
       Include.Attributes.Add("type", "text/javascript");
       Include.Attributes.Add("src", Page.ResolveUrl("~/Scripts/jlr.js"));
       this.Page.Header.Controls.Add(Include);

So now, I have to write a javascript function to handle my ddl, verify my
value
and then do a submit if it's necessary.

Well,

tks, I find my solution