Due in March 2020, but could be later as it has not been seen in preview yet, this is something you would have expected to work from the outset. Typically this feature is provided by independant vendors like Exclaimer – but now using Outlook Web Access and a cmdlet in O365, Microsoft will finally provide a similar feature.

We look forward to trying it out.

See the Microsoft Roadmap here
https://www.microsoft.com/en-us/microsoft-365/roadmap?filters=&searchterms=signature