how do i send sms from my secondary sim o dual sim i phone xsmax

Extracted below from --> Using Dual SIM with an eSIM – Apple Support

Send messages with iMessage and SMS/MMS

You can use iMessage or SMS/MMS to send messages with either phone number.* You can switch phone numbers before you send an iMessage or SMS/MMS message. Here’s how:

1. Open Messages.
2. Tap the New button, in the top right-hand corner of the screen.
3. Enter your contact's name.
4. Tap the current phone number.
5. Tap the number that you want to use.

* Additional fees may apply. Check with your service provider.