Eaton 3 way dimmer switch
The answers are all in the manual
An ordinary 3-way switch uses two travelers. A smart switch doesn't necessarily need two travelers, and this one doesn't. There is an accessory switch available to let you dim at both locations. But assuming you are replacing one 3-way switch with the dimmer and the other one is staying the same, you wire according to Two Location Control Using a 3-Way Switch.

Top Left = Neutral (white, connected to the existing white bundle)
Top Right = Hot (note, this must be true hot, not &quot;switched hot&quot;)
Bottom Right = Load, also known as &quot;switched hot&quot;.
Bottom Left = Traveler. Yes, just one traveler.

This is almost certainly doable, since you have the sometimes elusive neutral available in the box. But it may get tricky. Specifically, typical configuration is that one of the 3-way switches will connect to two travelers and hot, and the other to two travelers and switched hot. But this smart switch needs to connect to both hot and switched hot, and the ordinary 3-way switch needs to connect to hot.
Note: Major changes to original answer, now that OP posted a second picture.
It looks now like this is either:

Power -&gt; switch 2 -&gt; switch 1 -&gt; fixture

or

Power -&gt; switch 1 -&gt; switch 2 -&gt; fixture

(My original answer had power and fixture both connecting in the switch 1 box.)
Basic answer will still be along the lines of:

Use one traveler as a &quot;signal&quot;
Use the other (former) traveler as a way to get hot (not switched) between the boxes

But the specifics will vary depending on exactly what is going on. In particular, determination of which switch connects to power and which switch connects to the fixture.