"variable is not defined", but it exists

I am very new to windmill, so this could be an user error.
I try to access a user variable in a "Step Input" of a "Send POST Request (http)" action.

To reproduce (also see screenshots):

1. Create a user variable (

u/USER/test

u/USER/test

), not a secret.
2. Create new flow with "Send POST Request (http)" action.
3. Add

${variable('u/USER/test')}

${variable('u/USER/test')}

in the Step Input "url".
4. Press "Re-evaluate input step" in tab "Test this step".
5. Error message "Error evaluating url: variable is not defined" appears.

I didn't find any example in the documentation, how to use variables inside Step Inputs.
Just putting text directly into the input field "url" or using

flow_input

flow_input

does work.

Actual username in text and screenshots was replaced.

description: Example
edited_at: ""
edited_by: ""
extra_perms: {}
path: u/USER/example
schema:
  $schema: https://json-schema.org/draft/2020-12/schema
  properties: {}
  required: []
  type: object
summary: Example
value:
  modules:
    - id: a
      summary: Send POST Request (http)
      value:
        input_transforms:
          body:
            type: static
            value: {}
          headers:
            type: static
            value: {}
          url:
            expr: "`${variable('u/USER/test')}`"
            type: javascript
        path: hub/11643/http/send_post_request
        type: script

description: Example
edited_at: ""
edited_by: ""
extra_perms: {}
path: u/USER/example
schema:
  $schema: https://json-schema.org/draft/2020-12/schema
  properties: {}
  required: []
  type: object
summary: Example
value:
  modules:
    - id: a
      summary: Send POST Request (http)
      value:
        input_transforms:
          body:
            type: static
            value: {}
          headers:
            type: static
            value: {}
          url:
            expr: "`${variable('u/USER/test')}`"
            type: javascript
        path: hub/11643/http/send_post_request
        type: script

"variable is not defined", but it exists

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