How to use the secondary number to send SMS on iphone?

How does Apple's mobile phone send messages with a secondary number? You can also use the extended functions of Apple's mobile phone to send messages. Today, let me introduce you:

1. First, we open the information and click on the upper right corner to prepare the information.

2. Then enter the phone number of the recipient.

3. At this point, a green button will appear below, prompting whether to use the primary number or the secondary number.

4. We click the sub-signature button.

5. Write down the information below, and then click the green arrow to send.