How to auto-fill fields based on info in others?

I need to fill in all the fields, based on information in another one automatically …
Ex: Through the “Customer Code” fill in the automatic “Customer Type” …

What should I do ??

Current Controller

  def new (conn, _params)
    changeset = Account.change_order (% Order {})
    cod_client = Repo.all (from (c in Client, select: {c.fantasia_cliente, c.cod_client}))
    client_type = Repo.all (from (c in Client, select: {c.client_type, c.client_type}))

     render (conn, "new.html", changeset: changeset,
      condemnation, condemnation,
      sales_codes: sales_codes,
      customer_code: customer_code,
      client_type: client_type)
  end

Current Template

  <div class = "form-group">
    <label> Client Type
     <div class = "input-group">
    <span class = "input-group-addon" id = "basic-addon1"> <i class = "fa fa-angle-double-down" aria-hidden = "true"> </ i>
    <% = select (f, :client_questions, @cod_client)%>
    </ div>
    <% = error_tag f, :client_questions %>
    </ label>
  </ div>

  <div class = "form-group">
    <label> Client Type
     <div class = "input-group">
    <span class = "input-group-addon" id = "basic-addon1"> <i class = "fa fa-angle-double-down" aria-hidden = "true"> </ i>
    <% = select (f, :tipocli_pedidos, @client_type)%>
    </ div>
    <% = error_tag f, :tipocli_pedidos %>
    </ label>
  </ div>