The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
if (!this._isChoice) {
      throw new Error("Not a choice widget");
    }
    if (!cName) {
      return;
    }
    if (nIdx < 0 || nIdx > this.numItems) {
      nIdx = this.numItems;
    }
    if (this._items.some(({ displayValue }) => displayValue === cName)) {
      return;
    }
    if (cExport === undefined) {
      cExport = cName;
    }
    const data = { displayValue: cName, exportValue: cExport };
    this._items.splice(nIdx, 0, data);
    if (Array.isArray(this._currentValueIndices)) {
      let index = this._currentValueIndices.findIndex(i => i >= nIdx);
      if (index !== -1) {
        for (const ii = this._currentValueIndices.length; index < ii; index++) {
          ++this._currentValueIndices[index];
        }
      }
    } else if (this._currentValueIndices >= nIdx) {
      ++this._currentValueIndices;
    }
    this._send({ id: this._id, insert: { index: nIdx, ...data } });
  }
  setAction(cTrigger, cScript) {