Where have you added it in, exactly? In your fork? If so, you can use your fork in your mixfile.

# somewhere in your mix.ex
defp deps() do
  [
     # ...
     # can be something like this
     {:ex_crypto, github: "hoseinisalim/ex_crypto", branch: "my-function"}
     # ...
  ]
end

If you added your function in the original repo, then your update would be available once a new version of it is released on hex.pm

# somewhere in your mix.ex
defp deps() do
  [
     # ...
     # can be something like this (replace <some_new_version> with the actual version)
     {:ex_crypto, "~> <some_new_version>"}
     # ...
  ]
end

or maybe you can point mix to the repo’s master branch to use it now

# somewhere in your mix.ex
defp deps() do
  [
     # ...
     # can be something like this
     {:ex_crypto, github: "ntrepid8/ex_crypto"}
     # ...
  ]
end