Init Trim Sheet Material
The Init Trim Sheet Material tool can be found in the Trim Sheet panel of Blender's Material Properties.
Based on the selected Trim Sheet the tool initializes/creates a Trim Sheet Material for you.
Keep in mind
If the current selected Material is matchable, the new Trim Sheet Material will match that material, and will be named after it.
By default the tool will unwrap to the Empty Trim too, unless you prevent this by running it in ALT mode.
The Unwrap Trim tool has similar capabilities built in for convenience.
Selection
A material slot in object mode.
Some faces in edit mode.
Modes
ALT: Keep existing Trim UVs