Have you installed office setup in your computer?
You can Create an Email Signature in Office 365 by Using a Transport Rule.
To Create an Email Signature in Office 365 follow the given steps:
Make a text file of your Email Signature.
You need to Exchange admin center in the admin center by selecting Exchange.
Click on Rules in Mail Flow.
Create a new rule by click on + button.
Add the name of signature file, whatever you want.
In the bottom of window, Click on More Options.
Now select the desired candidates in the Apply this rule if menu.
Under the Do the following
drop-down menu, select Apply a disclaimer to the message and select append a disclaimer.
Copy & Paste the HTML code in the enter text field box.
Click on Save.
for more information, visit the official website : office.com/setup.