Очистить текстовое поле, используя Ctrl + A и Del (для Selenium IDE):
<tr>
<td>keyDown</td>
<td>id=your text field id</td>
<td>\17</td>
<tr>
<td>keyPress</td>
<td>id=your text field id</td>
<td>\65</td>
<tr>
<td>keyUp</td>
<td>id=your text field id</td>
<td>\17</td>
<tr>
<td>keyPress</td>
<td>id=your text field id</td>
<td>\127</td>