You can do this using Android Automate, an app I personally prefer to Tasker for most things. Here's a screenshot of the flow I used to trigger an alarm when a text is received from a specific phone number. Use this screenshot to see the order in which the blocks need to connect to each other.
And the details of the blocks:
This block waits for a text to be received. Set the "Phone number" field to the number you want to watch.
This is what we'll use to simulate the alarm. Set the "Proceed" option to "Immediately." This will ensure that the sound plays continuously until we manually stop it using another block. For good measure, check the "Repeat sound indefinitely" checkbox as well.
Set the "Notification channel" to a user-managed channel with "Urgent" priority. If you don't already have one, in the dialog that appears, click the plus button at the top of the dialog to create the new channel. You'll then need to save out of the block and flow, all the way back to the list of all flows. Then tap the hamburger menu at the top of the screen and click "Settings" > "Notification channels." Then tap on the name of the new channel you created. This will take you to your phone's settings for the channel. Set "Importance" to "Urgent", and turn on both "Vibrate" and "Override Do Not Disturb." Then go back to the Automate app and return to editing the new flow.
This displays a dialog that you have to manually close before we'll stop the sound.
Set the title and message options to whatever you wish, and check "Show window directly if possible."
This is what stops the alarm once you close out the dialog. It's important that you connect this block's "OK" circle to the "IN" circle of the "SMS received" block. Doing this ensures that the flow will loop and continue listening for SMS messages once an alarm is dismissed.
Once you've finished creating the flow, save it, rename it to what you wish, and click the "Start" button. It's also a good idea to go to the app's settings and enable "Run on system startup." This will ensure that the SMS alarms flow will resume immediately when you restart the phone.