Passing Value in Window Application

Your question is not clear, but i'm posting portion of code that may help you......

if (dataGridView1.DataSource != null)
{
    DataTable dt = (DataTable)dataGridView1.DataSource;
    if (dataGridView1.SelectedRows.Count <= 0) { return; }
    if (dt.Rows.Contains(dataGridView1.SelectedRows[0]))
    {
        DataRow[] dr = dt.Rows.Find(dt.Rows[0]["fieldname"]);
        if (dr.Length <= 0) return;
        //change the value of datarow
        dr[1] = txtName.Text; //  set related value
        dr[2] = txtcity.Text; //  set related value
        // now accept the change...
        dt.AcceptChanges();
        dataGridView1.Refresh();
    }
}

There are so many options... and yes, you should have provided more information :). Do you really have to use click event?
If yes - create a new event in your "edit" form, pass textbox's value as an argument and subscribe to this event in form with datagridview. Or you can pass a reference to your datagridview in "edit" form constructor/custom show method, though I strongly recommend you to avoid this practice at all costs.
If you just need to set value in your form - ie "edit" form is modal or may be made so - well, just return new text value via constructor/custom display method

Add this method in your form which contains DatagridView

public void populateGrid(string text)
{
DataGridViewRowCollection rows = this.dataGridView1.Rows;
rows.Add(text);
}

and call this method from the form which consists your textbox.

May b this can help you.