For loop counter and External function

Let's say my script begins with

-------------------------

a=input("Key in value: ")

For k=1:10

y=externalfunction(a)

end

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And my function

function[y]=externalfunction(a)

a=10+a

end

The error pops out because the counter k is not defined in the function. So how can I define it in the function so I can use it in the script? Please don't tell me it's unnecessary to use an external function. It's actually necessary and this is just an analogy to my far more complex question.