Azure AD custom rule to add character to existing attribute

I am migrating off ADFS to Azure AD and the vendor requires an M be added to the start of the employeeid attribute. In ADFS, I just wrote the custom rule Below:

c:[Type == "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/employeeID

What the claim then is, is M(and the employeeID) And it worked great. However, I can't figure out how to accomplish this in Azure AD. Any help would be greatly appreciated.

I've tried transformations but don't see the way.