From time to time, some will articulate the wish for missionary candidates to simply offer themselves to the agency and go wherever they are sent. This is typically viewed as a mark of spiritual maturity and surrender to Christ.
In general, I don't wholly agree with this. I suggest that you, the perspective candidate, should listen to it with a certain amount of caution.
Yes, we should be fully surrendered to Christ.
That is not the same as being fully surrendered and obedient to an agency, church or pastoral leader.
The calling of Christ is between Christ and the individual. It is a journey of discipleship, on which Jesus commanded us to follow Him. In following him on a path, we may find others are on the same path, at roughly the same spot we are. We are following Jesus together.
The agency, like the church, is a particular kind of fellowship or community. (I have elsewhere argued that, in a sense, an agency is a church, in that any gathering of believers is a church, because the believers together are the church.)
But while we are all part of a Body, the reality is, we have to pick and choose which individual community we are part of. We pick and choose this on the basis of the leading of Christ.
In short, you are on a path following Christ, and that path is defined by the command of Christ in your life. You need to pick an agency that is on the same path. Don't let an agency tell you which path you should be on--only Christ can tell you that.
Now, having chosen the agency, and the place you will be working, expect that there will be a great deal of daily sacrifice, surrender, and service to be done. If you are in a community, you are to be serving your brothers and sisters. I am not suggesting you get to define for yourself what you do on a daily basis! Jesus calls us to pick up our cross and follow him--to die daily to self, and live for others.