I have created a simple example. Here the select component reacts to manipulating FormControl inside component's class. Please try to use my example code to implement your feature.

HTML:

<form class="md-form">
  <mdb-select-2
    [multiple]="true"
    placeholder="Choose your option"
    label="Example label"
    [formControl]="selectControl"
  >
    <mdb-select-all-option>Select all</mdb-select-all-option>
    <mdb-select-option *ngFor="let option of options" [value]="option.value">{{
      option.label
    }}</mdb-select-option>
  </mdb-select-2>
</form>

TS:

import { Component, OnInit } from '@angular/core';
import { FormControl } from '@angular/forms';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {
  ngOnInit(): void {
    this.selectControl.setValue(['1', '2']);
  }

  selectControl = new FormControl(['1']);
  options = [
    {
      value: '1',
      label: 'one',
    },
    {
      value: '2',
      label: 'two',
    },
    {
      value: '3',
      label: 'three',
    },
  ];
}