Hi, I'm Stara. Unfortunately, the Phone Link app itself doesn’t have a built-in option to add a password or PIN just for launching it. What you can do is sign out of your Windows Account after each use so others cannot access your session. Creating a dedicated Windows User Profile for yourself can work as well. You can do this by following these steps:
- Open Settings > press Windows key + I.
- Go to Accounts > Family & other users.
- Under Other users, click Add account.
- If you want, you can add your Microsoft account. If not, select I don’t have this person’s sign-in information. Then Add a user without a Microsoft account.
- Create a username and password for your new profile.
- After it’s created, you can log in to Windows using your personal profile.
More info here https://support.microsoft.com/en-us/topic/phone-link-requirements-and-setup-cd2a1ee7-75a7-66a6-9d4e-bf22e735f9e3.