See Add an Android Contact or Add a Contact on an iPhone to learn how to add contacts to your phone.
Android: In the Settings app, tap Accounts and select Signal. Tap ⁝, then select Remove Account. Open Signal, tap ⁝ or ☰, and tap Refresh. The contacts will sync. [1] X Research source iPhone: Open Signal after adding the contact to your phone. Signal will automatically sync new contacts. [2] X Research source
The Signal desktop app must already be linked to Signal on your smartphone or tablet.