You should mean the setting of personalized signature, as long as you do the following:

(1) Open the QQ mailbox, enter the homepage of the mailbox, and "Settings" at the upper left of the motor:

(2) On the new page, select the "General" option:

(3) Drop down the page until you see the column of "Personal Signature":

(4) Select "Add Personalized Signature" and enter the text you want.

(5) When writing a letter, there is an option "Signature: Do not use" at the bottom of the letter page, so just use it instead.

thank you