Yes you have to use your US number to sign up for the US version of the NALA app.
After choosing US as the sender country, the prefix for US numbers will be automatically provided for a seamless experience.
But this also means that you can only sign up to use the app if you have a valid US number. Once they have entered your phone number, you will receive a code to authenticate it. Enter the code in the app and press continue to complete the sign up process.