Disable Angular Reactive form input based on selection value

You'll can listen to the valueChanges event of the form :

this.paymentForm.valueChanges.subscribe((value) => {
  if(value.paymentMethod == 'opt-1'){
   this.paymentForm.controls['testInput'].disable();
  }else{
   this.paymentForm.controls['testInput'].enable();
  }
});

So everytime the select changes , the valueChanges event is called , the conditions kick in and it will enable or disable the formControl.

You can leverage the selectionChange @Output property on MatSelect and react accordingly:

onSelectionChanged({value}) {
  console.log(value);
  if(value === 'opt-1') {
    this.paymentForm.get('testInput').disable();
  } else {
    this.paymentForm.get('testInput').enable();
  }
}

And in template

<mat-select ... (selectionChange)="onSelectionChanged($event)">

Here's a Sample StackBlitz for your ref.

NOTE: In case there are more controls in your form than just mat-select, listening to valueChanges on the whole form could be expensive as this will get triggered every time there is a change in any of the form control. All we are concerned about is the change in the mat-select selection change.

Though there are already answers provided, If any had stumbled with the same issue. You can directly disable a form field by directly accessing its control

Had created a Stackblitz demo link

  <!-- INPUT FIELD --> 
  <mat-form-field formControlName="testInput">
    <input matInput 
           placeholder="Some input"
           [disabled]="paymentForm.get('paymentMethod').value === 'opt-1'">   // Disables the input once paymentMethod's formControlName value is opt-1    
  </mat-form-field>