As Andre explained, you need to first understand how the music industry works and who are the major decision makers.

Again, Andre’s answer gives you an overview, but you can find more detail here Music Industry Landscape | Kythera | Understand the Business of Music

Back to your question now.

How you can negotiate opening slots is through showing your work and approaching the right people.

Showing your Work -

This includes Videos, Photos, Fan Reviews, Recordings etc. This should be easy as all of this will already be in your EPK, or atleast it should be.

Approaching the Right People -

Figure out who are your ideal Bands that you want to open for, or Festivals that you want to perform at. Find the person of contact at both of these and pitch them about your band and music. This can be done through email or phone. Cold calling people works, but a reference from another agent or company will give you a huge boost.

Be persistent about approaching these people. It will be difficult and you’ll hear a lot of No’s, but if they see your persistence they’ll start respecting you and give you the slot.

But the most important thing is you should approach the bands that really match your sound and have a similar target audience. No one will consider you if they find a disconnect at this basic point.