Respected Sir,
I am very interested in joining this amazing campaign and I would be grateful if I get the opportunity to be part of this campaign. I promise that I will be a high quality poster for the campaign. I am willing to accept half pay per post, for example $1.5 per post, instead of the full amount of $3 per post, in order to prove my commitment and dedication to the campaign.
Thank you.
Why are you requesting to join at half pay rate? If you are eligible to participate in this campaign and fulfill all their requirements then the manager will be willing to accept you at full pay rate. So I think expressing interest to participate in this campaign at this half pay rate will not bring positive results for you. So don't give up hope, keep posting good posts and you will be able to join other campaigns at a decent pay rate according to your rank. Even in this campaign, maybe one day you will be accepted.