Mashfiqun, you do not need to be focused just on how you are going to join to some signature campaign, especially when you are just Jr. Member and by that very limited by the number of campaigns that accept such members. It's a lot better to read&learn more, and to try post quality posts which will surely bring more merits for you. Most of campaigns have the conditions in the form of merits, although there are those who do not attach great importance to it.

I joined my first signature campaign just at the time when I reach Full Member rank, so you have time - forum will go nowhere