add the checkbox to the insert and update page. set the checked value to 1.
On the insert page, edit the Insert record behavior and bind the position column to the checkbox.
On the update page, edit the update record behavior and bind the position column to the checkbox.
On the update page, you will also need to edit the Update page recordset to include the position column in the select statement.
then select the checkbox in design view, in the property inspector, click the Dynamic Button. for the check if options, click the lightning bolt and select the position column form the recordset, set the equal to option and enter 1.