I don’t suggest using SetText because it will be not compitable with other programs else.

Suggested Solutions:

• TextBox Property:

Add in your user control TextBox Property and then use it.

Code sample: In UserControl.cs

public TextBox TargetTextBox { get; set; }

public void SetText(string text)
{
    TargetTextBox.Text = text;
}

And use new method that are avaliable in UserControl

• Event:

Add event in the user control, and if this event fired in Form1, use SetText in code there.

Sample Code: Create file called TextEventArgs and put this code:

public class TextEventArgs : EventArgs
{
    public TextEventArgs(string text)
    {
        Text = text;
    }
    public string Text { get; set; }
}

Use this code in user control

public delegate void ShowTextEventHandler(object sender, TextEventArgs e);
public event ShowTextEventHandler ShowText;

// Use this method, and this method will fire 'ShowText' event
protected override OnShowText(TextEventArgs args)
{
    TextEventHandler handler = ShowText;
    handler?.Invoke(this, args);
}

• Variable (Not suggested):

Go in Program.cs file and put replace it with this code:

using System.Windows.Forms;

public class Program
{
    internal static readonly MainForm;

    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompitableTextRenderer(false);
        MainForm = new Form1();
        Application.Run(MainForm);
    }
}

And then you can use Program.MainForm.SetText("example").