You can refer to the NDA in the "Welcome Message" portion of the test setup and say that "By continuing you accept our [URL link to terms]". You can use Markdown to add links to the Welcome Message field.

If you want to see the participant sign the contract, include the contract signing link as the optional "Landing Page" of the test setup. This way they'll start on that page and have to sign in order to continue. You'll then have a recording of them signing the contract in the case that is helpful. 

If your process requires that the participant accept terms or NDA before the test starts, we recommend adding your NDA to the set of instructions that you email to your participant, along with the Participate URL for them to join the session.