is there a way to make your own charter code.🤔
You can't make your own, it's automatically generated. Once you've declared independence from your mother colony (and you're playing an online colony) your charter code will be active. You can see what it will be (if you haven't declared independence) by looking at the URL for your colony in the statistic overview screen.
Example:
One of my colonies has the URL https://www.my-colony.com/colonies/FV8keiG4/
The last bit "FV8keiG4" is the charter code.