When you 'cite and close', the citation will be added to the document. As Authorea is a writing environment, the formatting of the citation will not match your style but it does not matter because when you will export your Authorea document to other formats, you can control the citation styling. 
Here is an output as an example of this approach:
Torkamani and Topol (2019) have recently expanded the scope of polygenic risk scores to obesity \cite{Torkamani2019}
I suppose in 80% cases, this will be sufficient for you to add citations. If that does not work, you can specify a bibtex file. Let's move to step 2. Search for a reference on Google Scholar and generate a bibtex file (see the screenshots):