 <p>You can either accept the customParent as a argument in the constructor, or better yet, the <code>FindBox</code> form should take a <code>Action&lt;string&gt;</code> that it will call once the find button is pressed.</p> <h2>Example:</h2> <pre><code>class MainForm : Form { public void FindNext(string find) { // Do stuff } public void OpenFindWindow() { FindBox find = new FindBox(this.FindNext); find.Show(); } } class FindBox : Form { private Action&lt;string&gt; callback; public FindBox(Action&lt;string&gt; callback) { this.callback = callback; } public void FindButtonPressed() { callback(textBox1.text); } } </code></pre>