click button to start new interface.

Pop-up window is not possible on button click but it will be helpful if you can share the exact requirement here. But as you want to navigate between 2 screens, you can use flag functionality. On this create a local variable and on click of the button set the value of the local variable to 1 or anything and then use showwhen functionality of if else functionality.

a!localVariables(
  local!show:true(),
  
  {
    if(
      local!show,
      {
        a!richTextDisplayField(
          value:a!richTextItem(
            text:"Clicked"
          )
        )
      },
      {
        a!richTextDisplayField(
          value:a!richTextItem(
            text:"Un-Clicked"
          )
        )
      }
    ),
    a!buttonLayout(
      a!buttonWidget(
        label:"Click",
        submit: false(),
        saveInto: {
          a!save(local!show,not(local!show))
        }
      )
    )
  }
)

If you want to use it in process model then please have a look on activity chaining concept.

Pop-up window is not possible on button click but it will be helpful if you can share the exact requirement here. But as you want to navigate between 2 screens, you can use flag functionality. On this create a local variable and on click of the button set the value of the local variable to 1 or anything and then use showwhen functionality of if else functionality.

a!localVariables(
  local!show:true(),
  
  {
    if(
      local!show,
      {
        a!richTextDisplayField(
          value:a!richTextItem(
            text:"Clicked"
          )
        )
      },
      {
        a!richTextDisplayField(
          value:a!richTextItem(
            text:"Un-Clicked"
          )
        )
      }
    ),
    a!buttonLayout(
      a!buttonWidget(
        label:"Click",
        submit: false(),
        saveInto: {
          a!save(local!show,not(local!show))
        }
      )
    )
  }
)

If you want to use it in process model then please have a look on activity chaining concept.