How to validate only one field in a Flutter form?

I have a form with several fields, at the end of the form there is a Login button which validates the whole form before submit. Within the form there is a "Send Code" button which requires only the email address as mandatory field. How can I just highlight the email address field without highlighting the password field when the Send Code button is pressed?

Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(20.0),
          child: Form(
              key: _formKey,
              child: Column(children: [
                Row(children: [
                  Expanded(
                    child: TextFormField(
                      decoration: InputDecoration(
                          border: OutlineInputBorder(),
                          labelText: 'Email Address'),
                      controller: _EmailAddressController,
                      validator: (value) {
                        if (value == null || value.isEmpty) {
                          return 'Please enter email';
                        }
                        return null;
                      },
                    ),
                  ),
                  Expanded(
                    child: TextButton(
                      onPressed: () {
                        if (_formKey.currentState!.validate()) {
                          ScaffoldMessenger.of(context).showSnackBar(
                            const SnackBar(content: Text('Processing Data')),
                          );
                        }
                      },
                      child: Text('Send Code',
                          style: TextStyle(
                              fontSize: 24,
                              color: Colors.green[800],
                              fontWeight: FontWeight.bold)),
                    ),
                  )
                ]),
                SizedBox(height: 10),
                TextFormField(
                  obscureText: true,
                  decoration: InputDecoration(
                      border: OutlineInputBorder(), labelText: 'Password'),
                  controller: _PasswordController,
                  validator: (value) {
                    if (value == null || value.isEmpty) {
                      return 'Please enter password';
                    }
                    return null;
                  },
                ),
                TextButton(
                  onPressed: () {
                    if (_formKey.currentState!.validate()) {
                      ScaffoldMessenger.of(context).showSnackBar(
                        const SnackBar(content: Text('Processing Data')),
                      );
                    }
                  },
                  child: Text('Login',
                      style: TextStyle(
                          fontSize: 24,
                          color: Colors.grey[800],
                          fontWeight: FontWeight.bold)),
                ),
              ]
              ),
          ),
        ),
      ),
    );
  }