JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="email1" value="@test.com" class="domain-fixed" data-domain="@test.com">

<input type="text" name="email2" value="@exemple.fr" class="domain-fixed" data-domain="@exemple.fr">

JavaScript

const domainFixedInputs = document.querySelectorAll('.domain-fixed');

domainFixedInputs.forEach(function(input) {
  input.addEventListener('keydown', function(e) {
    if (e.key == '@') {
      e.preventDefault();
      return false;
    }
  });

  input.addEventListener('keyup', function(e) {
    let parts = this.value.split('@');
    this.value = parts[0] + this.dataset.domain;
  });

  input.addEventListener('focus', function(e) {
    let parts = this.value.split('@');
    this.setSelectionRange(parts[0].length, parts[0].length);
  });
});