No, if you needed to do that, I would've said so.
You do not need to change the code at all. You need to change the value (HTML value attribute) of the Select All checkbox to an asterisk. Only that.
Can you tell me where I can find the HTML Value? Sorry, I'm new to marketo.
All I did was dragged the HTML from the elements panel onto the landing page and pasted the above the code in the Custom HTML editor in the landing page.
I'm not sure where I can change the HTML Value.
On a Marketo-hosted LP you should not be using the embed code nor the MktoForms2.loadForm() method.
You should be using a named Form element (it's right there on the right side of your screenshot).
Make sure your HTML Element is placed after (below) the Form element. Do not include the loadForm() line in your HTML element.
Ryley, Have one of the options be "all of the above". Then have a smart campaign in the background check all those boxes on the contact's record if they choose the "select all option". Hope that helps!