Creating an Inputbox in C# using forms

In your main form, add an event handler for the event Click of button Add Ip Address. In the event handler, do something similar as the code below:

private string m_ipAddress;
private void OnAddIPAddressClicked(object sender, EventArgs e)
{
    using(SetIPAddressForm form = new SetIPAddressForm())
    {
        if (form.ShowDialog() == DialogResult.OK)
        {
            //Create a property in SetIPAddressForm to return the input of user.
            m_ipAddress = form.IPAddress;
        }
    }
}

Edit: Add another example to fit with manemawanna comment.

private void btnAddServer_Click(object sender, EventArgs e)
{
    string ipAdd;
    using(Input form = new Input())
    {
        if (form.ShowDialog() == DialogResult.OK)
        {
            //Create a property in SetIPAddressForm to return the input of user.
            ipAdd = form.IPAddress;
        }
    }
}

In your Input form, add a property:

public class Input : Form
{
    public string IPAddress
    {
        get { return txtInput.Text; }
    }

    private void btnInput_Click(object sender, EventArgs e)
    {
        //Do some validation for the text in txtInput to be sure the ip is well-formated.

        if(ip_well_formated)
        {
            this.DialogResult = DialogResult.OK;
            this.Close();
        }
    }
}