This is not an advert sent by or related to Textra SMS, this a Class 0 message sent to you by your carrier / service provider. They are messages, that typically are not kept, so appear as just a pop-up, like alerts mostly. Spammers are now sending and using this type of SMS message.

To assist on the above either SAVE this message first and then use Textra's Blocklist feature (long holding this chat > 3 dots on the top right > Blocklist). Alternatively, contact your carrier / service provider to block Class 0 messages sent for you.