How to select all the option in the multipledropdown field by default and also provide "select all" option in dropdown

You can pre-populate a local variable with the the choice value array data and map this variable in value and saveinto of the dropdown. When the form loads your local will have all the dropdown values in it. And the dropdown will also show all the values pre-selected. Users can then deselct/select the values as needed. 

a!localVariables(
  local!language: {
    "en_US",
    "es_ES",
    "fr_FR",
    "de_DE"
  },
  a!multipleDropdownField(
    label: "Language",
    instructions: "Which language(s) are you proficient in?",
    choiceLabels: {
      "English",
      "Spanish",
      "French",
      "German"
    },
    choiceValues: {
      "en_US",
      "es_ES",
      "fr_FR",
      "de_DE"
    },
    value: local!language,
    saveInto: local!language,
    placeholder: "Select a language",
    searchDisplay: "ON"
  )
)

Hey

This code helps achieve the functionality you're looking for using a multiple dropdown. All the options are stored in a local variable and preselected by default. To deselect, you can simply click the “×” icon on the multiple dropdown field, which clears the selection. This gives a clean and user-friendly way to toggle selections without extra UI controls.

yeah thanks for the code. but what if user want to select all the options again. customer want option as select all and deselect all along with the options

Well in that case try this code ,this can be a workaround the intended functionality

a!localVariables(
  local!allOptions: {
    1, 2, 3, 4
  },

  local!selectedOptions: local!allOptions, /* Default: all selected */

  local!selectAll: true, /* Controls checkbox */

  {
    a!checkboxField(
      label: "Select All",
      choiceLabels: {"All"},
      choiceValues: {true},
      value: if(count(local!selectedOptions) = count(local!allOptions), {true}, {}),
      saveInto: {
        a!save(
          local!selectedOptions,
          if(
            or(a!isNullOrEmpty(save!value) , length(save!value) = 0),
            {}, /* Deselect all */
            local!allOptions /* Select all */
          )
        )
      }
    ),

    a!multipleDropdownField(
      label: "Options",
      choiceLabels: {"Option 1", "Option 2", "Option 3", "Option 4"},
      choiceValues: local!allOptions,
      value: local!selectedOptions,
      saveInto: {
        a!save(local!selectedOptions, save!value)
      }
    )
  }
)