[help] how to insert rom file in my computer into webMSX from the start?

please, help.
i want to turn on webMSX and start automatically a rom file in my computer.
for example,i tried like below.

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CARTRIDGE1_URL: "C:\\MSXrom\\zanac_ex.rom"
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and an error comes out like this.

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Could not load file: C:\MSXrom\zanac_ex.rom
Error: 0
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what's wrong with me?
please teach me right ways.