Let us assume that you have decided to add your marketo form to a new source or a website. This becomes a new lead source. Will your Marketo system automatically populate this field for the new leads arriving through the source?
Marketo forms will not automatically update this field. If you want this field populated, you will need to manually set this up within a Smart Campaign to update, or, have this set to a hidden field on the form itself.
It is acquisition program details that will update automatically, but only if a Marketo form is submitted on a Marketo landing page.
What if the Marketo form was embedded inside another page instead of the market LP? Let us assume some kind of social media platform. Twitter? Facebook?
What happens then?
I think your best bet is to capture the lead source within a Smart Campaign (SC). Ideally, you'd have a global SC that is looking for an identifier in the form name e.g. if Form Name contains SOCIAL then Lead Source = Social Media.