Material not displaying correctly in custom mesh.

So I built a custom mesh using a quick tutorial online. It works fine if I use just a basic material on it but if I use anything with a texture, that texture doesn't translate. Here is an example: https://imgur.com/a/LG9gLZJ

In that picture, both the triangle and the plane beneath it have the exact same material. It shows up correctly on the plane, but not on the triangle. If I change the material offset the triangle changes color, so I'm guessing it's some issue with the way the texture is projected onto the mesh? If I change the MeshFilter mesh to "Plane," I can then see the texture but of course it overrides my custom shape and turns the triangle into a square.

Can someone tell me what I am overlooking?

 

No UV coordinates I presume.

 

Yeah, you're right. If I put in some UV coordinates I start to see the pattern, though it's all messed up. I'm not sure how to calculate them.

EDIT: I found some docs that had an example. After figuring out how to correctly calculate the UVs, the material is working properly. Thank you @mouurusai !

The doc is at https://docs.unity3d.com/ScriptReference/Mesh-uv.html , in case anyone comes across this later and needs it.