You can use css to hide it. Inspect the entry form in the browser to see how the input elements of the url field are rendered. In recent versions it's a table with the label and element of the url text as the second row.
Here is a simple illustration of the add template:
<div class="defaulttemplate"><table class="mod-data-default-template">
<tbody><tr class="r0">
<tr class="r1 lastrow">
<td class="template-field cell c0" style="">URL: </td>
<td class="template-token cell c1 lastcol" style="">URL</td>
</tr>
</tbody>
</table>
</div>
Enclose the field tag in the add template with an identified div:
<div class="defaulttemplate"><table class="mod-data-default-template">
<tbody><tr class="r0">
<tr class="r1 lastrow">
<td class="template-field cell c0" style="">URL: </td>
<td class="template-token cell c1 lastcol" style=""><div id="id_myurl">URL</div></td>
</tr>
</tbody>
</table>
</div>
Add css in the css tab of the activity:
#id_myurl table tr:nth-child(2) {
display: none;
}
hth 