Hello, what email address are you using? For example, in the QQ mailbox, you can click Settings at the top left, find the personalized signature in the General tab, and click Add Personalized Signature to set the signature. And multiple signatures can be set.

If you have multiple accounts, you can also set different signatures for each account. Find the "Account Management" below, click Settings to the right of the account name, select the account to use its nickname and signature, and then select one of the multiple signatures you set before.