How do I call upon a dependent dropdown field?

Hi. If you need to display data from the "Storage Location" table that depends on the "Plant" table, the "Storage Location" table should include a field that serves as a foreign key referencing plantId in the "Plant" table.
Please refer to this link to configure cascading dropdowns: https://docs.appian.com/suite/help/25.3/recipe-configure-cascading-dropdowns.html

Use the selected plantId to lookup its corresponding value field from plantData, then filter storageData where werks matches that value to populate the dependent Storage Location dropdown.

a!localVariables(
  /* Sample Plant Data */
  local!plantData: {
    a!map(plantId: 1, value: "P100", description: "Plant Chicago"),
    a!map(plantId: 2, value: "P200", description: "Plant Dallas"),
    a!map(plantId: 3, value: "P300", description: "Plant Boston")
  },
  /* Sample Storage Location Data */
  local!storageData: {
    a!map(werks: "P100", name1: "Warehouse A", land1: "USA"),
    a!map(werks: "P100", name1: "Warehouse B", land1: "USA"),
    a!map(werks: "P200", name1: "Storage Unit 1", land1: "USA"),
    a!map(werks: "P200", name1: "Storage Unit 2", land1: "USA"),
    a!map(werks: "P300", name1: "Depot North", land1: "USA"),
    a!map(werks: "P300", name1: "Depot South", land1: "USA")
  },
  /* Variables to store selections */
  local!selectedPlantId: null,
  local!selectedStorageLocation: null,
  /* Get the value field of selected plant */
  local!selectedPlantValue: if(
    a!isNullOrEmpty(local!selectedPlantId),
    null,
    index(
      local!plantData.value,
      wherecontains(local!selectedPlantId, local!plantData.plantId),
      null
    )
  ),
  /* Filter storage locations based on selected plant */
  local!filteredStorageLocations: if(
    a!isNullOrEmpty(local!selectedPlantValue),
    {},
    index(
      local!storageData,
      wherecontains(local!selectedPlantValue, local!storageData.werks),
      {}
    )
  ),
  /* Form Layout */
  a!formLayout(
    titleBar: "Plant and Storage Selection",
    contents: {
      /* Plant Dropdown */
      a!dropdownField(
        label: "Plant",
        placeholder: "-- Select Plant --",
        choiceLabels: local!plantData.description,
        choiceValues: local!plantData.plantId,
        value: local!selectedPlantId,
        saveInto: {
          local!selectedPlantId,
          a!save(local!selectedStorageLocation, null)
        },
        required: true
      ),
      /* Storage Location Dropdown - FIXED */
      a!dropdownField(
        label: "Storage Location",
        placeholder: "-- Select Storage Location --",
        disabled: a!isNullOrEmpty(local!selectedPlantId),
        choiceLabels: if(
          a!isNullOrEmpty(local!filteredStorageLocations),
          {},
          local!filteredStorageLocations.name1
        ),
        choiceValues: if(
          a!isNullOrEmpty(local!filteredStorageLocations),
          {},
          local!filteredStorageLocations.name1
        ),
        value: local!selectedStorageLocation,
        saveInto: local!selectedStorageLocation,
        required: true
      )
    }
  )
)

Please refer to this link to configure cascading dropdowns.

https://docs.appian.com/suite/help/23.3/recipe-configure-cascading-dropdowns.html