You can use RowDataBound event for that below code sample for your reference
protected void mainGridView_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow &&
(e.Row.RowState == DataControlRowState.Normal ||
e.Row.RowState == DataControlRowState.Alternate))
{
if (e.Row.Cells[1].FindControl("selectCheckbox") == null)
{
CheckBox selectCheckbox = new CheckBox();
selectCheckbox.ID = "newSelectCheckbox";
e.Row.Cells[1].Controls.Add(selectCheckbox);
}
}
}