The client portal functions (fka communicate) more like a secure instant messenger than an email client. In that sense, your username is present in every communication channel used. Your identity and that of your recipient is implied in the communication channel itself therefore the use of an email signature is not necessary.

If you still desire to include a signature on your messages in the client portal you might try a service such as TextExpander which allows you to use shortcodes in typing to duplicate text. For example, “Jonathan Kibler” was typed by me with “;name”.