Here is some code that will get an IWpfTextViewHost reference on the C# interactive Window. From there, you can have access to all text services from Visual Studio: Text lines, Text buffer, etc. (or you can hook directly on WPF's controls, which I don't recommend)

// get global UI shell service from a service provider
var shell = (IVsUIShell)ServiceProvider.GetService(typeof(SVsUIShell));

// try to find the C# interactive Window frame from it's package Id
// with different Guids, it could also work for other interactive Windows (F#, VB, etc.)
var CSharpVsInteractiveWindowPackageId = new Guid("{ca8cc5c7-0231-406a-95cd-aa5ed6ac0190}");

// you can use a flag here to force open it
var flags = __VSFINDTOOLWIN.FTW_fFindFirst;
shell.FindToolWindow((uint)flags, ref CSharpVsInteractiveWindowPackageId, out IVsWindowFrame frame);

// available?
if (frame != null)
{
    // get its view (it's a WindowPane)
    frame.GetProperty((int)__VSFPROPID.VSFPROPID_DocView, out object dv);

    // this pane implements IVsInteractiveWindow (you need to add the Microsoft.VisualStudio.VsInteractiveWindow nuget package)
    var iw = (IVsInteractiveWindow)dv;

    // now get the wpf view host
    // using an extension method from Microsoft.VisualStudio.VsInteractiveWindowExtensions class
    IWpfTextViewHost host = iw.InteractiveWindow.GetTextViewHost();

    // you can get lines with this
    var lines = host.TextView.TextViewLines;

    // and subscribe to events in text with this
    host.TextView.TextBuffer.Changed += TextBuffer_Changed;
}

private void TextBuffer_Changed(object sender, TextContentChangedEventArgs e)
{
    // text has changed
}

Note "Microsoft.VisualStudio.VsInteractiveWindow" assembly is not specifically documented but the source is open: http://sourceroslyn.io/#Microsoft.VisualStudio.VsInteractiveWindow