<div class="containeer">
  <h1 mat-dialog-title>How to add your Telegram account</h1>
  <div mat-dialog-content class="content">
    <span
      >To get your UserId, you have to write "/id" or "/auth" to the bot on
      Telegram. (<a href="https://t.me/projektaktienbot)"
        >https://t.me/projektaktienbot)</a
      >)</span
    >
  </div>
  <div mat-dialog-actions class="form-group footer-buttons">
    <div class="inner">
      <button
        id="okButton"
        class="btn btn-secondary btn-block"
        (click)="close()"
        [mat-dialog-close]="true"
      >
        Ok
      </button>
    </div>
  </div>
</div>

./help-dialog.component.scss

.footer-buttons {
  width: 100%;
  text-align: center;
}

.spacer {
  flex-grow: 1;
  width: 5%;
}

.inner {
  display: inline-block;
  width: 100%;
}

.content {
  height: 80%;
}