Can't grab the select element inside iframe in cypress

I have a select element inside a iframe and need to select an option in a cypress test. But it doesn't find the element. Here is the html:

<iframe id="KBasePopUp_frame_iframe" onload="KBasePopUp_frame.onIFrameLoaded(this)" scrolling="auto" width="550" height="230" frameborder="0" src="https://bmw-rsa-asd-int.kcenter.usu.com/knowledgebase/docCreate.do?ctrl.returnPage=closeInfoBoard.nav&amp;showMenu=false&amp;ctrl.nested=true&amp;appAreaId=f0d159c8-19e5-4a2a-825f-19fe4f553686&amp;query.docTypeClass=*&amp;categorySelection=on" style="height: 230px;" bis_size="{&quot;x&quot;:710,&quot;y&quot;:550,&quot;w&quot;:550,&quot;h&quot;:230,&quot;abs_x&quot;:710,&quot;abs_y&quot;:550}"></iframe>

and the select element is somewhere down below:

<select name="query.docTypeId" onchange="markDataChanged();" class="largeSelectBoxWarn" title="Mandatory field, please insert data!">
    <option value="4002" title="Automatic question">Automatic question</option><option value="3001" title="Privacy Policy">Privacy Policy</option>
    <option value="1" title="Problem">Problem</option>
    <option value="12" title="Solution">Solution</option>
</select>

I don't have access to the developer's code and can only inspect with web tools.

As @spender mentioned, interacting with an iframe is described in Cypress's blog.

You have to first retrieve the iframe document, and then its body, and then query from there.

const getIframeDocument = () => {
  return cy
  .get('#KBasePopUp_frame_iframe')
  // Cypress yields jQuery element, which has the real
  // DOM element under property "0".
  // From the real DOM iframe element we can get
  // the "document" element, it is stored in "contentDocument" property
  // Cypress "its" command can access deep properties using dot notation
  // https://on.cypress.io/its
  .its('0.contentDocument').should('exist')
}

const getIframeBody = () => {
  // get the document
  return getIframeDocument()
  // automatically retries until body is loaded
  .its('body').should('not.be.undefined')
  // wraps "body" DOM element to allow
  // chaining more Cypress commands, like ".find(...)"
  .then(cy.wrap)
}

it('clicks the select option', () => {
    getIframeBody().find("select.largeSelectBoxWarn option[value=1]").click();
});

Making it more flexible

The above is hard coded to your specific iframe since it's just supposed to teach you a concept. However, you could make it more flexible by accepting the query to the iframe as a parameter to getIframeBody('#KBasePopUp_frame_iframe');

const getIframeDocument = (iframeQuery) => {
  return cy
  .get(iframeQuery)
  // Cypress yields jQuery element, which has the real
  // DOM element under property "0".
  // From the real DOM iframe element we can get
  // the "document" element, it is stored in "contentDocument" property
  // Cypress "its" command can access deep properties using dot notation
  // https://on.cypress.io/its
  .its('0.contentDocument').should('exist')
}

const getIframeBody = (iframeQuery) => {
  // get the document
  return getIframeDocument(iframeQuery)
  // automatically retries until body is loaded
  .its('body').should('not.be.undefined')
  // wraps "body" DOM element to allow
  // chaining more Cypress commands, like ".find(...)"
  .then(cy.wrap)
}

it("clicks the select option n KBasePopUp iframe", () => {
  getIframeBody("KBasePopUp_frame_iframe").find("select.largeSelectBoxWarn option[value=1]").click();
});