Hi @anabananei,
Do you want to send an email with attachment of type .msg? You could add it as attachment in the email.
While if you want to send an email with the link of an email, please get the URL of the email from Outlook Web, then format the link likes below:
<a href="https://outlook.office.com/owa/?viewmodel=ReadMessageItem&ItemID=AAMkADcwYTExNzc2LWFmOTMtNDY2ZC1iNWY1LWI0ZTkyZWQ5ODM5ZgBGAAAAAACaZteUoEZqRrRPN%2BTp9vrVBwDfjzNN4LzORaaQOsIp81uzAAAAAAEMAADfjzNN4LzORaaQOsIp81uzAACAz9neAAA%3D&wid=85&ispopout=1&path=">email </a>
I have made a test on it and the email can be checked. Please try it on your side.

Note: Make sure you have selected Yes for the Is HTML field.
Best regards,
Mabel Mao