[Rockin]

Ya lo he consegui de la siguiente forma, por si le sirve a alguien:

Código Delphi [-]
Function FillForm(WebBrowser: TWebBrowser; FieldName: string; Value: string): Boolean;
var
i, j: Integer;
FormItem: Variant;
begin
Result := False;
//no form on document
if WebBrowser.OleObject.Document.all.tags('FORM').Length = 0 then
begin
Exit;
end;
//count forms on document
for I := 0 to WebBrowser.OleObject.Document.forms.Length - 1 do
begin
FormItem := WebBrowser.OleObject.Document.forms.Item(I);
for j := 0 to FormItem.Length - 1 do
begin
try
//when the fieldname is found, try to fill out
if FormItem.Item(j).Name = FieldName then
begin
FormItem.Item(j).Value := Value;
Result := True;
end;
except
Exit; 
end;
end;
end;
end;

procedure TfrmPrincipal.CmdRellenarClick(Sender: TObject);
begin
        FillForm(WebBRowser, 'user', 'usuario');
        FillForm(WebBRowser, 'platformcode', '123456');
        FillForm(WebBRowser, 'password', 'XXXX');
end;

Saludos.