Example: (the following snippet of text with citation was added using this approach)
Dominigue et al (2014) have suggested that polygenic risk scores can predict obesity in both Blacks and Whites \cite{domingue2014polygenic}
You can also use a citation manager such as Endnote, Mendeley, and Zotero to generate bibtex files of the papers that you want to cite, and copy paste the bibtex code to the window that results from clicking 'add citations from reference managers' and work this way.
Last, but not the least, you can store a bibtex file (a file with file extension '.bib' in Authorea and pull in citations. Generate a bibtex file from your favourite reference management system. Then, in Authorea, click on 'Document v' (the first drop down element on the left) , then select 'data folder'. In the data folder, select 'bibliography' folder and within it locate a file named 'biblio.bib'. Authorea draws its citation information from there. You can either replace the biblio.bib with your own generated bibliography (bibtex formatted) file or append entries to it. Then, click on the 'cite' button and search on the search box but make the search 'local'. Authorea will locate the reference and you continue the steps to insert the citation. The following image shows a locally generated bibtex file and Authorea's biblio.bib side by side. I pulled in the citations from my local bibtex file and appended them to Authorea's biblio.bib, and then searched locally for the following reference.