How does Xiaomi mobile phone send SMS to add personalized signature?

The first thing to know is that Xiaomi mobile phone has a system setting key, which is the one with three horizontal lines in the lower left corner of Xiaomi screen.

Generally speaking, all programs can use this key to open the settings menu. As for your question,

Solution:

Open the SMS menu interface →→→→→ Press the system setting key →→ Turn on the additional signature switch →→ Edit the signature → Save and complete OK. Useful to praise. Thank you.