Adding this here to save folks some time. I have been working with CrushFTP for over 4 years and the addition of DUO integration could not have come sooner. Passwords are dead! I have been testing the functionality and got it to work. Here is what you will need.
Setup a DUO account. You must use a paid account (you can trial one for 30 days). DUO MFA, DUO Access or DUO Beyond. You must use these in order to access the Auth API.
Once you first get into the account click on Applications and "Protect Applications" Find the "Auth API". Click it and it will provide you with integration and secret keys. Within the Crush Admin --> Preferences --> plugin dialog select CrushDuo plugin. You will need to copy 3 elements from the Duo screen. These are the API URL, Integration Key, and Secret Key. Click save in Crush. (NOTE: you may want to start out with a single user in the plugin to get everything working and prevent lockout of your admin account).
Within the DUO admin portal under "protect application" make sure you SAVE your settings. I got so excited copying over elements to put into crush I never hit save and spent 30 min troubleshooting. Once saved you will need to create users under your account. I created user names that matched crush users and sent auth requests to each user to setup the DUO app on their phone. I added phone numbers, e-mail, and potential aliases (including e-mail) to the user just in case, but I am still not sure what is used to tie a crush user to a duo user. But it works...
Make sure you save everything; and give it a try.