Do you mean to link which landing page the prospect goes once they fill in the form? If so - just go to the landing page editor and then double click on the form. It should open up a pop up like below. Then if you select Landing Page for the follow up type, and then select your Acknowledgement page in the Follow Up page drop down box.
Hope that helps!
If the above is not what you are looking for, you can check out - Add a Rich Text to a Form - Marketo Docs - Product Docs. Essentially you'll enter in a hyperlink within a rich text block on the form.