This kind of interview is more like cooperation. This will be more like a meeting for joined forces so to speak. You have a common goal, to get you into the work market.
The bureau is not going to hire you if they are not able to sell you to another employer. If they are able to, you will earn money and they will earn money from you. You have a common interest. So in this interview you do not need to sell yourself, but you may point out important facts, give them arguments to be able to sell you to others, explain your expectations, your goals, your requirements and your flexibility. Listen well to their reply and reply accordingly.
As this is a much more informal interview, you may even ask for advice for your further presentation and ongoing process to become part of the job market. The consultant bureau may point out details that should be clearer in your CV. The salary is also often an open issue in this setting. Here you may explain your requirements and be agreed on the salary that will be right for you.
Some bureaus keep the rate asked for as the customer secret for you (their candidates), but if you are willing to ask for a lower salary to get into the work market, let them know you expect the bureau to lower the rate too.
The consultant bureau will write your CV on their form and will sell you using positive language. Be clear with them that they should always inform or ask you about the positions they are applying for, as you should not apply for the same jobs. They will be fine with you applying for jobs yourself too.
When the consultant bureau finds an interested employer it is up to you to take it further in an interview process with the future employer.