From Elm docs you will find:
every : Time -> Signal Time
lift : (a -> b) -> Signal a -> Signal b
get : String -> Request String
send : Signal (Request a) -> Signal (Response String)
Above functions can be used to do what you need:
send $ lift (\_ -> get myURL) $ every (10 * seconds)
which will be of type Signal (Response String)
I haven't tested the code but I hope this gives you the idea.