The even you probably want to use is
TextBox1_Validated
That event means that any validation that needs to occur on the TextBox has been completed and the contents of the TextBox are valid.
You absolutely do
not want to use
TextBox1_TextChanged
because you will end up with a new row for every
keystroke in TextBox1!
[EDIT]
The above solution assumed that you had a textbox separate to the DataGridView and entering data into it would prompt a new row being added to the DGV.
OP has now indicated that they are entering data into the first textbox column of the DGV itself and want to add a row at that point.
As far as I can determine you cannot do this as the act of editing a row temporarily prevents new rows being added. There is a list of the events that are affected on this link
DataGridViewRowCollection.Add Method (System.Windows.Forms)[
^]
An alternative is to have an "input area" for the DGV - you could potentially position a textbox over the DGV and then use the technique I originally suggested.