1)You can create public property in UserControl like:
public string SomeString
{
get;
set;
}
And then pass string by xaml:
<UserControl1 SomeString="String"/>
or
bind SomeString property to property in your MainWindow
<UserControl1 SomeString="{Binding MyStr}"/>
2)You can pass your string to UserControl by constructor.
var uc = new UserControl(MyString);
solved How to pass data from WPF form to WPF usercontrol? [closed]