How to access a user control in a masterpage from a content page?

Dim myMaster as MyMasterPage = CType(Page.Master, MyMasterPage)
myMaster.MyUserControl.Text = "Hello!"

UserControl uch = Page.Master.FindControl("ucHeader1") as UserControl;
PlaceHolder phProxylist= ucHeader1.FindControl("phProxy") as PlaceHolder;
DropDownList ddlproxylist1 = ucHeader1.FindControl("ddlProxyList") as DropDownList;
phProxylist.Visible = false;