First, you need to create one. It should be an image of 64x64 pixels or less.

Then you would need to create a Single Texture Directive of this image:

1. Open Rexmech generator[rexmeck.github.io] or any other generator of your liking,
2. Select your image,
3. Press Generate

IMPORTANT!
You only need the directives: do NOT copy the /assetmissing.png part of the result. Copy everything starting with the first ❓ character

When you get the one long line of directives, paste them in the text field and press the Set button