Steps on how to set that up in Fastmail are below:
Go to your Settings in Fastmail:
Click on New App Password:
Choose Custom from the first drop down and give it a name, then select SMTP from the second drop down, then select Generate password.
Copy that new password to your Elements Form Settings >> Password field.
Enter your details, along with the app specific password into the Elements Form component:
Publish your site to test your contact form is working. if you run into any problems please feel free to post over on the Elements Forum for further help.