Toggle Angular checkbox with string values instead of boolean true/false

I have to create three checkboxes in my form and their values need to toggle as "optin" and "optout" (string) as I check/uncheck the checkbox. Also I need to check all the checkboxes by default for "optin" value. I cannot figure out any solution, any help would be really appreciated :( as I am fairly new to angular

Here is the code I am working on

component.html

<form [formGroup]="optOutForm" (ngSubmit)="submitOptFormValue()">
    <div class="form-group">

   <input type="checkbox" formControlName="optOutFlag1" id="privacy" aria-labelledby="check1">
   <label for="privacy" id="check1"> Checkbox 1</label><br><br>

   <input type="checkbox" formControlName="optOutFlag2" id="security" aria-labelledby="check2">
   <label for="security" id="check2"> Checkbox 2</label><br><br>

   <input type="checkbox" formControlName="optOutFlag3" id="consent" aria-labelledby="check3">
   <label for="consent" id="check3"> Checkbox 3</label><br><br>

   <button> Submit Preference </button>

</div>
</form>

component.ts file

optOutForm:FormGroup
this.optOutForm=this.fb.group({
    optOutFlag1:["optin",Validators.required],
    optOutFlag2:["optin",Validators.required],
    optOutFlag3:["optin",Validators.required]
});