Insert: Decrescendo    ⌃⇧.

Using this command is so much easier than finding the decrescendo in the part box and dragging it to the location. Simply click at the right spot, press the keys, presto.

I have observed that for a software instrument (Classical Piano) a meta event on MIDI channel 2 is inserted — Crescendo — for both crescendo and decrescendo. The length is indicated in bars and beats. I have no idea how (or if) the direction is indicated or used.

Add dynamic marks, slurs, and crescendi to a score in Logic Pro — Apple Support

You can add slurs to indicate notes that should be played legato, and add dynamic crescendo and decrescendo symbols (sometimes called hairpins). Automatic slurs can cover a selected group of notes, and adjust when the notes are copied, moved, or transposed. Using key commands, you can quickly add and edit automatic slurs for an entire phrase, part, or score.