Form fields have two boolean configs, validateOnBlur and validateOnChange. If you set these to false, fields won't be validated during user input. In this case you have add a code for example to your submit button, to check for validation errors. Ext.form.Basic has isValid and hasInvalidField methods that you can call, the first marks validation errors, the second does not.

@Peter Koltai has the correct answer for classic.

For modern you should use:

    validateTextfield(field) {
        test...
        if(!valid){
            field.setErrorMessage('bad decision');
        }
    },
    onBeforeSubmit() {
        const form = this.getView(),
            values = form.getValues(),
            isTextFieldValid = this.validateTextfield(values.fromTextField);
        return isTextFieldValid;
    }
    onSubmit() {
        const isValid = this.onBeforeSubmit();

        if(!isValid) return;

        doMySubmission();
    }