Insert Textbox value to a specific row and column?

Question

Good day,

 

    I have a problem inserting a data or a text from textbox in a specific column and a specific row in datagridview without database using add button.

 

I have a code her but the problem of this code it insert string text in all the column selected.

 

I want only one row in a specific column should be inserted...please help..thank

 

 

 
for (int i = 0; i < DatagridVoucher.Rows.Count; i++)
{
DataGridViewRow row = DatagridVoucher.Rows[i];
row.Cells[3].Value = this.txtVat.Text;
}

 

 

I'm attaching file for more details thank you .