These commands (after the first one) work in the ed editor. ed editor is used in the RStudio server shell.

sudo crontab -e                                     #opens crontab file in chosen editor

a                                                   #add to file
0,30 * * * * Rscript /home/user/folder/script.R     #command to add
.                                                   #finished editing
,p                                                  #print file content to check
w                                                   #save changes
Q                                                   #quit

In this example the script will run every 0 hours, 30 minutes, see Ubuntu: How do I set up a CRON job for other options.