To access the Request File button in the CentreStack Android app, do the following.
From MyFiles, navigate to the folder location where you are going to request a file from a guest user. Then long-press (1) the folder where the file will be saved. From the pop-up context menu, choose Share (2). Then within the Share view, click the Request File icon (3) and enter the email address and password (4) for the user that will be sending you the file you are requesting. Press the green send icon (5) to finish.
That user will receive an email (6). You must give them the password (7) so they can access the folder.
Once they have uploaded the file. It will soon sync to the folder where you requested the file (8) in your CentreStack Android app.