Hi
There are other members in this community who know much more about this and me, but I'll do my best to recite the advise I have
seen provided here previously. If Fox_M is around, he'll show you all the tips and tricks and then some. Until then, I can share with the the basics
First, you need to add a new layer and choose 'generic' from the menu

In the next menu available, choose 'image'

Now that you have it set to image, you can add your image source to the zone.
You can add whatever you like here, even video.
In your case, you want to add polka dots. You can add a png, jpeg etc that you imported. Or, you can make your own in Motion, probably with a replicator.
If you are adding your own source that you made in Motion, you need to have that source in it's own group, and be sure that you have set that group to be fixed resolution.

If you do use a replicator, then make sure you set that to be 3D. It is not because the text is 3D, but it will make the shapes - the dots in this case - clear and sharp. Using a 2D replicator can make images fuzzy, depending on the project. ( I don't now why exactly, but I learned the rule of thumb is that a 3D replicator makes images sharp)
Once you have your source set, you can make adjustments for the size and position with the menu options.

Those are the basics I can pass on to you.
When you use a source as a texture for 3D text make sure that source is independent of other groups and that it runs the entire duration of the project. Otherwise, you might find the texture disappears, or does not comply when layers are moved forward and back in the time line. This is especially so for video sources - ..for a still image or motion layers, I am not sure sure. But when you start animating the texture source - then this is something to keep an eye on. I'm not even 1% sure about how the relationship between the 3D text and the texture source - there are others in the community here that can help with that.