To build this app you need to have a Reddit account.
Once you have that, head over to this page to get a client id for the app.
Make a new installed type app with appropriate information.

Now on your development machine goto

• Windows: C:\Users\<Your Username>\.gradle
• Mac: /Users/<Your Username>/.gradle
• Linux: /home/<Your Username>/.gradle

and then add the following 2 lines to gradle.properties file (create one if it doesn’t exist).

UPDOOT_CLIENT_ID = "xxxxxxxxxxxxxx"
UPDOOT_REDIRECT_URI = "https://redirecturl.com"

where xxxxxxxxxxxxxx is the client id that you got after creating a new installed type app
and https://redirecturl.com is the redirect url spicified while creating the installed type app.

For further information on Reddit’s oauth usage use this guide.