test.html

<form class="form">
  <select name="select" id="mySelect">
    <option default value="choice1">Choice One</option>
    <option value="choice2">Choice Two</option>
  </select>
  <button type="submit">Submit</button>
</form>

<form class="form">
  <select name="select" id="mySelect2">
    <option default value="choice1">Choice One</option>
    <option value="choice2">Choice Two</option>
  </select>
  <button type="submit">Submit</button>
</form>

<script>
  let plans = {
    mySelect: {
      choice1: "https://mylink.com",
      choice2: "https://myotherlink.com"
    },
    mySelect2: {
      choice1: "https://my3rdlink.com",
      choice2: "https://my4th.com"
    }
  }

  //let select = document.querySelector("#mySelect")
  let forms = Array.from(document.querySelectorAll(".form"))

  forms.map(form =>
    form.addEventListener("submit", event => {
      event.preventDefault()

      const formSelect = event.target.querySelector("select")
      window.location.assign(plans[formSelect.id][formSelect.value])
    })
  )
</script>