Agile is not a method, is not a process, did not start with the Manifresto. Agile is based on strategy. So, the answers you need are here:Rick Dove´s "Response Ability" book and all related work. Saving Changes...
Do you want to explain it to them, or do you want to help them see value in the idea?
You are competing with many other people, ideas, and conflicting priorities for their attention.
It's not just about giving the information. They have to be open to receiving it, and they have to be open to receiving it from you. If you don't have the kind of relationship with them that makes that possible, and you aren't in a position to develop that relationship, find someone who does and work with that person.
Is your message specific, or are you just telling them that strategic planning is important and they should do it? If the latter is the case, you're pretty much just telling them they are failing because they are not doing what you think they should do. At least, that is one way your message could be perceived. Make sure your message communicates what you want it to communicate.
Does the information you want to present demonstrate you've done your research? Strategic planning is not just about strategic planning. It's about innovating and identifying ways to deliver value. There are different ways to accomplish these things, like the three horizons framework. But you have to be careful. There are some things your company might need to stop because they don't deliver value, but if you just come out and tell them this, it might be like hitting a hornet's nest with a baseball bat.
Another consideration is the increasing rate of change many companies are seeing. Some company's used to create 5 and 10 year strategic plans. For most company's, this does not work any more; the rate of change is too fast. I, personally, believe that you sometimes have to slow down in order to speed up, but most people seem to get caught up in the rate of change and try to make their cadence match. It's not sustainable.
If you can, find out what your company has done with strategic planning, in the past. What has your management's experience been with it?
In short, know your audience, know your subject matter, refine your message, build relationships with your audience, get a champion if needed. This is one approach; there may be others that work better in your situation. Saving Changes...