Where do you set the sector tax rate?

I'll give a little more elaboration on taxing sectors and give you a reference.

Note: Since the most recent update 1.7.4 this screen looks a tad different but it functions mostly the same.

The portion I highlighted is what determines your tax rate. All you have to do is click on it and you can swap between taxing rates, the max being 75%. You can see the +(Minerals/Credits) number go up and down as you alter the tax rate so you can see how much income you're taking from them. That number is how much the sector is earning. The yellow number is how much they currently have. You'll also be able to see your income rates increase/decrease as you alter the tax rate.

As of the newest updates, you can now select an option next to the sectors resources to spend influence and basically withdraw credits/minerals from the sector when they've got an excess lying around.

The button that shows credits/minerals going into that hexagon bit, next to the sectors resources, is how you put resources into the sector (to help them develop usually). Those are in increments of 100.

Each of the buttons in between are the different management tools. You can change the focus of the sector and allow them to redevelop tiles or not. (Military, Minerals, Balanced and Science).

According to this Steam community thread:

Right above i think it is manage sector is a little button that lets you increase the amount. 50% is default.

This reddit question places that button in the same menu:

So the % you can adjust in the sector hub, called sector contribution, is the tax rate[...].