A very simple method to include an opt-in list form can be accomplished by utilizing the "Create an Opt-in Page" tool under the advertiser dashboard and the "Custom HTML" widget within the Premium Mobile Website builder.
Begin by navigating to the "Create an Opt-in page" tool under the advertiser dashboard
Select "Website" for the "Create a Code for:" value and select the keyword associated to the desired opt-in list that you wish for the end-user to be part of. Input a header, sub header, and check any additional fields you wish to include. Click the "Generate Code and Preview" button at the bottom when finished
Navigate to the Premium Mobile Website page that you wish to edit, then drag the "Custom HTML" widget into the editor. An edit window will pop up. Simply paste the custom HTML into this editor.
Click "Save" on the Edit Custom HTML window and finally "Save" on the page editor to finalize the addition.