Yesod: how do you interpolate a route with a path piece?

Question

If I have a route such as

/foo/#String FooR GET POST

How do I interpolate this path in a hamlet file (say, to post to?)

The following don't work:

<form method=post action=@{FooR} ... >
<form method=post action=@{FooR}someString ... >
<form method=post action=@{FooR}#someString ... >
<form method=post action=@{FooR}#{some_variable} ... >

Thank you!

EDIT: Got it. It's

<form method=post action=@{FooR some_variable} ... >