Hi @Ke Ku ,
An MS ID is used by Pearson VUE to locate your account, you will only have an MSID if you have completed an exam previously, to find your MSID as follows:
1. Login to your Microsoft Certification dashboard: https://www.microsoft.com/en-us/learning/dashboard.aspx
2. Under Exam providers, select Pearson VUE and then click Go.
3. You will now be redirected to Pearson VUE's site, at the left side click on the arrows to expand your left-hand panel:
4. You should now see your MSID, as follows:
Please also note that Microsoft Certification is currently not supported in the Q&A forums, the supported products are listed over here https://learn.microsoft.com/en-us/answers/products.
If your still having issues finding it, you can contact the Pearson VUE support over here:
----------
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Best regards,
Leon