The phrase "for agreeing" is correct and usable in written English.
It can be used to express gratitude for someone agreeing to do something. For example: "Thank you for agreeing to help me with the project."
Thank you, Ms. Haney, for agreeing with all my suggestions!
BANKSY: Thanks for agreeing to play my theme park.
Thank you for agreeing to meet with me.
Pimps had several reasons for agreeing to the interviews.
Nye has been criticized for agreeing to this debate.
But the bar should be set very high for agreeing to honor those requests.
Officials for the league and the players union commended Braun for agreeing to the suspension.
Canadians received less than an eighth of this amount for agreeing to "retire their rides".
The entrepreneurs cite various reasons for agreeing to take on an apprentice.
Candidates who take part may receive matching funds in return for agreeing to limits on spending.
Madonna collects her award for agreeing to show up for the H.F.P.A.'s clambake.
