Open your HTML email signature file in your web browser.
Press Ctrl + A to select the entire email signature, then press Ctrl + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Ctrl + A as you may miss out on grabbing some of the code.
Open Outlook and select the Settings cog at the top right of your screen.
Select Accounts, then Signatures.
Enter a name for your email signature, then click inside the signature's text field below and press Ctrl + V to paste your copied email signature. Click Save.
At the bottom of the window, set your email signature as default for New Messages and Replies/Forwards. Click Save again.
Close out of your Settings and compose a new email. Your email signature will now automatically insert when composing emails in Outlook Office 365 on Windows.