I was not able to add it as standard sms app with the proposed manifest changes.
However manifest change worked for me:
[CODE lang="b4x" title="manifest"]AddApplicationText(
<!-- BroadcastReceiver that listens for incoming SMS messages https://stackoverflow.com/questions/30127564/how-do-i-set-my-app-as-the-default-sms-app -->
<receiver
android:name=".SmsReceiver"
android

ermission="android.permission.BROADCAST_SMS">
<intent-filter>
<action android:name="android.provider.Telephony.SMS_DELIVER" />
</intent-filter>
</receiver>
<!-- BroadcastReceiver that listens for incoming MMS messages -->
<receiver
android:name=".MmsReceiver"
android

ermission="android.permission.BROADCAST_WAP_PUSH">
<intent-filter>
<action android:name="android.provider.Telephony.WAP_PUSH_DELIVER" />
<data android:mimeType="application/vnd.wap.mms-message" />
</intent-filter>
</receiver>
<!-- Activity that allows the user to send new SMS/MMS messages -->
<activity android:name=".ComposeSmsActivity">
<intent-filter>
<action android:name="android.intent.action.SEND" />
<action android:name="android.intent.action.SENDTO" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.BROWSABLE" />
<data android:scheme="sms" />
<data android:scheme="smsto" />
<data android:scheme="mms" />
<data android:scheme="mmsto" />
</intent-filter>
</activity>
<!-- Service that delivers messages from the phone "quick response" -->
<service
android:name=".HeadlessSmsSendService"
android:exported="true"
android

ermission="android.permission.SEND_RESPOND_VIA_MESSAGE">
<intent-filter>
<action android:name="android.intent.action.RESPOND_VIA_MESSAGE" />
<category android:name="android.intent.category.DEFAULT" />
<data android:scheme="sms" />
<data android:scheme="smsto" />
<data android:scheme="mms" />
<data android:scheme="mmsto" />
</intent-filter>
</service>
)[/CODE]