quite simply, you need to add this to your PlayerWindow file:
Code:
<Label item="GP_Name">
<ScreenID>GPName</ScreenID>
<Font>2</Font>
<RelativePosition>true</RelativePosition>
<Location>
<X>3</X>
<Y>0</Y>
</Location>
<Size>
<CX>79</CX>
<CY>14</CY>
</Size>
<EQType>1</EQType>
<Text>Playername123346</Text>
<TextColor>
<R>255</R>
<G>255</G>
<B>255</B>
</TextColor>
<AlignLeft>true</AlignLeft>
<NoWrap>false</NoWrap>
</Label>
You can Change the Item and ScreenID value to what ever you want. I just copied it from my mod and I use GP_ as a self identifier of my custom additions.