that's tdac you talking about? Also what visa you applying? Tourist or non-o? If tourist maybe you have to say you staying that many days you intend of staying and if you for fill rest of requirements shouldn't be an issue. Good luck
How my wife did it 'Hello her name invite my husband your name to stay with me on adress from the date you land. Kind regards her name' you might have to include a photo of the bluebook
lol, many different type schengen visa not only tourist visa.
And OP asked for how to prove his relationship. If you read my comment further down, I responded. She need to show strong ties to return to Thailand, many pictures, chat log, call log and more.