Lead Source is a required selection within the contact section when creating a Listing or Offer.
Click on the indicated line to open the lead source menu of options. There is a list of lead sources available so scroll down or start typing to find the one you are looking for.
Lead source options with an *asterisk next to it will populate a field below for you to provide more details. Click on the 'Please Specify' line and begin typing to add details to your selected lead source.