Thanks for reaching out! My name is, Zadee. I'm an Independent Advisor and a Microsoft user like you. I'll be glad to help you today.
I understand that you want to add another MS account to the Phone link app.
You're only asked once to login your MS account during the setup. And if you're already using an MS account to login to your windows device, it might actually just auto detect that. If you need to change that MS account, you may have to reset the app through Settings > Apps > Apps installed.
For more info about the Phone link app, please refer to this link:
https://support.microsoft.com/en-gb/topic/phone...
Thanks!