<div class="form-outline">

  <input type="text" id="custom-mask" class="form-control" data-mdb-clear-incomplete="false" data-mdb-input-mask="27ZG            " 

        data-mdb-custom-mask=" " data-mdb-custom-validator="[A-Za-z0-9]*"/>

  <label class="form-label" for="custom-mask">Custom mask</label>

</div>

​

const inputEl = document.querySelector('input.form-control');

​

inputEl.addEventListener('focus', () => {

  const splitString = inputEl.value.split('27ZG');

  splitString[1] = splitString[1] ? splitString[1] : '';

  inputEl.value = '27ZG' + splitString[1];

})