Try giving your Button in your main.xml a more descriptive name such as:
<Button
android:id="@+id/buttonXYZ"
(use lowercase in your xml files, at least, the first letter)
And then in your MainActivity class, declare it as:
Button buttonXYZ;
In your onCreate(Bundle savedInstanceState) method, define it as:
buttonXYZ = (Button) findViewById(R.id.buttonXYZ);
Also, move the Buttons/TextViews outside and place them before the .setOnClickListener - it makes the code cleaner.
Username = (EditText)findViewById(R.id.Username);
CompanyID = (EditText)findViewById(R.id.CompanyID);