saveToMicroservice add variable

How can i add set a variable in the WF i execute with the saveToMicroservice service? I want to define the variables value within Apps.
If there is now way, i guess i have to use the saveToFunction service as a wrapper?

Does adding "variableAssignments": [] in the config of your workflow element not help?
Selecting the variable via a filter.

sth like this:

      "id": "XXX_endpoint",
      "type": "workflow",
      "async": true,
      "config": {
        "workflowId": "00000000-0000-0000-0000-000000000000",
        "@workflowId": {
          "partial": "id_workflow_trigger_XXX"
        "projectId": "00000000-0000-0000-0000-000000000000",
        "@projectId": {
          "partial": "id_project"
        "variableAssignments": [
            "variableName": "my_var",
            "variableType": "STRING",
            "variableValue": "",
            "$variableValue": {
              "value": "{{filters.select_var.my_var.values.0}}"

Or you can also add this ‘variableAssignment’ to a table element → config → editing → saveToMicroservice