@Nguyen Ba Tung You need to add template variable #dropdown to the element with mdbDropdown directive and then use ViewChild like this:

@ViewChild('dropdown') dropdown: MdbDropdownDirective;

HTML:

      <div mdbDropdown class="dropdown" #dropdown>
        <button
          class="btn btn-primary dropdown-toggle"
          type="button"
          id="dropdownMenuButton"
          aria-expanded="false"
          mdbDropdownToggle
        >
          Dropdown button
        </button>
        <ul mdbDropdownMenu class="dropdown-menu" aria-labelledby="dropdownMenuButton">
          <li><a class="dropdown-item" href="#">Action</a></li>
          <li><a class="dropdown-item" href="#">Another action</a></li>
          <li><a class="dropdown-item" href="#">Something else here</a></li>
        </ul>
      </div>

TS:

  @ViewChild('dropdown') dropdown: MdbDropdownDirective;
  constructor() {}

  ngOnInit(): void {}

  ngAfterViewInit(): void {
    Promise.resolve().then(() => {
      this.dropdown.show();
    });
  }