Within the plot details of any given plot, there will be a "Waiting list" feature ready for use. This allows you to keep track of customers that are interested in a property, as well as contact information to reach out to these individuals.
In order to add a customer to the plot waiting list, you will need to navigate to the customer's contact and scroll down to see "Properties interested in". Filling this out with a specific development and plot will reveal a checkbox labelled "Add to waiting list". Checking this box and clicking the green "Add interest" box beneath will add the information to the contact profile, as well as provide the customer's contact information within the desired plot's waiting list.