FME server webhooks: passing geojson polygon-coordinates in URL

Hi, taking my first steps in the FME REST API and webhooks, and struggling on passing coordinates.

I published a simple workspace to FME server using the Geometry Pïcker as only parameter, the drawn polygon is noted in GeoJson, like:

{"type":"Polygon","coordinates":[[[3.732869,51.016178],[3.733537,51.016688],[3.734896,51.015998],[3.734086,51.015473],[3.732869,51.016178]]]}

 I created a webhook from this workspace, the URL preview shows the special characters should be encoded, like:

https://FMEserver/workspace.fmw?DrawnPolygon=%7B%22type%22%3A%22Polygon%22%2C%22coordinates%22%3A%5B%5B%5B3.732869%2C51.016178%5D%2C%5B3.733537%2C51.016688%5D%2C%5B3.734896%2C51.015998%5D%2C%5B3.734086%2C51.015473%5D%2C%5B3.732869%2C51.016178%5D%5D%5D%7D&opt_showresult=false&opt_servicemode=sync  

but, when I GET-request through this URL, the workspace fails:

55	2021-12-7 17:05:35 | A JSON syntax error was found at line 1, column 2
56	2021-12-7 17:05:35 | Unexpected character: 't'. One of the following characters was expected: '"', '}'
57	2021-12-7 17:05:35 | The value of the import attribute '__GeometryReplacerTemp__' contained invalid JSON, and could not be parsed
58	2021-12-7 17:05:35 | The below feature caused the translation to be terminated

Looks like the polygon is not decoded correctly, as it is missing the "-characters?

62	2021-12-7 17:05:35 | Attribute(encoded: UTF-8) : `_DrawnPolygon' has value `{type:Polygon,coordinates:[[[3.732869,51.016178],[3.733537,51.016688],[3.734896,51.015998],[3.734086,51.015473],[3.732869,51.016178]]]}'
 
72	2021-12-7 17:05:35 | GeometryReplacer_<Rejected> (TeeFactory): GeometryReplacer_<Rejected>: Termination Message: 'GeometryReplacer output a <Rejected> feature. To continue translation when features are rejected, change 'Workspace Parameters' > Translation > 'Rejected Feature Handling' to 'Continue Translation''

Am I missing something? Should I encode/decode in a different way?

At the end, I hope to get input from a user through a custom webmap (ArcGIS Online and/or Leaflet), and use webhooks/RESTAPI to launch FME server workspaces, so there might be more convenient ways to do this, just trying to understand what goes wrong for now...