Are you talking about in your profile? If that options doesn't exists then you can't set that up.

Assuming that you're using Thunderbird, please see : https://support.mozilla.org/en-US/kb/signatures If you can't find what you're looking for in this article, then you can post your question on the Thunderbird support forum (still assuming that you ''are'' using Thunderbird) : https://support.mozilla.org/en-US/products/thunderbird (click on 'Ask a question' at the top of the page)