Disable Input fields in reactive form

Question

I already tried to follow the example of other answers from here and I did not succeed!

I created a reactive form (ie, dynamic) and I want to disable some fields at

Answer 1

this.form.enable()
this.form.disable()

Or formcontrol 'first'

this.form.get('first').enable()
this.form.get('first').disable()

You can set disable or enable on initial set.

 first: new FormControl({disabled: true}, Validators.required)