Simply click Add next to the desired reference.  You will then be given the option to Change Citation Text which allows you to select from a variety of in-text citation options (e.g., suppress author, remove parentheses) or simply to Cite and Close.   Once added, Authorea will automatically make a reference list at the end of your working document and update the list each time a new citation is added.   References will appear in a generic format in the reference section.  Note that these will appear in a generic Authorea style, not formatted to a particular citation style.