Is there an automatic way you add it to the end of your comments? As far as I can see it’s not possible in the Boost for Lemmy app on Android…
That will require a signature field, which the Lemmy web client doesn't have, and I'm not aware one way or the other for any of these Android / Apple clients.
What I did was I sent myself a text message, and then whenever I want to use it I just long press hold which copies and then a long press paste in the Lemmy web client editor.