How can I change default SMS app?

[healinghands33]

I have changed my default SMS app from GoSMS to Signal on my phone (Samsung Galaxy S5, Android 6.0.1). Whenever I sent messages from GoSMS and did a Sync, those messages showed up in my MPE desktop client.

However, now when I send a text message from the phone and refresh my Conversations in the MPE desktop client, the new Signal SMS doesn't appear in the list. Next I tried sending a text message from within the MPE desktop client and it was successfully received by the other person, who then replied to me. On my phone, however, my SMS TO that person appeared within the old GoSMS message list, but the reply FROM that person appeared in the new Signal list.

How can I change it so that the outbound desktop MPE messages also appear in the Signal SMS message list, along with the inbound messages?

Thanks in advance for any help you can offer.