The first thing you need to know is that Xiaomi mobile phones have a system setting button, which is the three horizontal lines in the lower left corner of the Xiaomi screen.

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

Solution:

Open the SMS menu interface →→ press System settings key→→Turn on the additional signature switch→→ Edit signature→→ OK after saving. Thank you for the good review if it is useful.

The answer is not easy. I hope it can help you. If you are satisfied, please help to adopt it. Thank you