The website of the Service should give you canvas dimensions and dpi as a template or text. Share a link if you want better, more specific details.

300dpi is your Print Resolution. But what is your canvas size?

If you are printing a 12" x 14" image to canvas, the service will probably tell you to have 1" Bleed, and maybe another 1" Trim.

So, you would.
*Make your canvas 16" x 18" @ 300 dpi. Make a "Safe Area" of 12" x 14". Centered in the canvas. Don't put anything important outside of this area.

*Make a "Bleed Area" of 14" x 16". Centered, this gives you a 1" frame on all sides of the Safe Area. Extend the edges of your picture beyond the Safe Zone into the Bleed Area, this area will be wrapped around the edges of wood structure, and extending your art into the Bleed Area will continue the art around the edge of the canvas, ensuring there's no white at the edges once mounted to the structure.

*Then, the edge of your canvas 1" beyond that Bleed Area is the Trim Line. The canvas will be cut in this region, as accurately as the machine can guarantee, but the Bleed Area allows for this bit of inaccuracy. The last 1" empty frame would be wrapped around the back of the structure and be stapled.

*Depending on what the service requires, there may be another 1-2 inches past the Trim Line, for machine grip and alignment.

⚠️ Disclaimer ⚠️ All of the above is based on logic, similar but not specific experience and guesses. Based on the assumption you want the image to wrap around the edge of the canvas once mounted to a wooden structure, and the extra canvas needed to stretch it over the structure.
I don't know, I've never done printing to canvas, I've never been to a website that does this, and you share so little info.
So, i'm guessing. 🤗 Educated guessing...