It’ll be easier (and more “proper”, see below) if you just run your script as
myprogram /path/to/file
Then you can access the path within the script as $1 (for argument #1, similarly $2 is argument #2, etc.)
file="$1"
externalprogram "$file" [other parameters]
Or just
externalprogram "$1" [otherparameters]
If you want to extract the path from something like --file=/path/to/file, that’s usually done with the getopts shell function. But that’s more complicated than just referencing $1, and besides, switches like --file= are intended to be optional. I’m guessing your script requires a file name to be provided, so it doesn’t make sense to pass it in an option.