Here are two (2) different strategies in accomplishing this goal.
Not Accepting Responses On the responses tab in Google Forms, click the slider "Accepting Responses" this will change the banner to red "Not Accepting Responses"
1. Go to the Google Forms editor and add a text field (short answer or paragraph). Call it “Password” and make it a required question.
2. Click the snowman (bottom right corner of this question and select Data Validation - Regular Expression - Matches
3. Type your password in the "pattern" field Leave custom error text empty
4. Next, insert a new section. This is the second page where you can add your regular form fields. No one can see this section unless they know the password.