marshaj

If you prefer to work that way, there are ways to do so.
 
One example:
 
Make sure you don't have any layers selected, then hit the Mask Layer button along the bottom of the Layers panel.
Use a black brush (or other relevant tools) to paint onto that layer the area you want to effect (if at 100% opacity you are asking for full effect and the area will turn transparent).
After you have the desired area targeted, invert the mask layer (Command+I by default on the Mac - Layer->Invert) and only the targeted area will be visible.
Deselect the mask layer, add the desired adjustment layer, drag it below the mask layer, and group the mask and adjustment layers together - you can then edit the adjustment layer and it will have an effect on the intended area.
If you select the mask layer, you can continue to refine the area.
If you add an additional adjustment layer to the group (keeping it below the mask layer) it will apply its effect to the same area as the existing one.

Hi, @marshaj. I think that part of the issue here is that "adjustment brushes" such as one finds in Lightroom and other apps do things in the opposite order than one finds in Affinity Photo. With an adjustment brush, you create the mask first (by "painting" with the adjustment brush) and then apply the adjustment settings to the masked area (usually designated with a red overlay). In Affinity, you create the adjustment (and likely its settings) first by creating the adjustment layer and using the sliders; then, you invert the adjustment layer and "paint" the adjustment back in where you'd like it. It's the same process, really, just done in a different order.

First of all thank you again!
With additional help from @NotMyFault I've finally managed to do the adjustment with the brush:
- Open image
- open new adjustment/live filter layer
- Perform adjustment on the entire image
- level - invert
- Set paintbrush to "white".
- paint over object to be adjusted
Now only the selected object is modified

It’s all there out of the box:
add adjustment or live filter layer invert layer (inherent mask) use regular brush to paint in adjustment where you want it

In regards to the advice above, it’s worth noting that Invert Layer is only available in Photo.
One alternative in Designer or Photo is to select the Adjustment Layer in the Layers Panel, (in Designer go to the Pixel Persona), and use the Flood Fill Tool with Black to ‘erase’ all of the adjustment.
You can then paint with White, or shades of grey, onto the adjustment layer.
Another alternative in Designer or Photo is to right-click the icon to the left of the Adjustment layer thumbnail (if there is a thumbnail, if not it’s the icon to the left of the name), and choose Edit Adjustment from the menu.
You can then use the Flood Fill Tool (in Designer you need to be in the Pixel Persona) with Black to ‘erase’ all of the adjustment.
Select any other layer in the Layers Panel (or select the canvas) to stop editing the adjustment.
You can then paint with White, or shades of grey, onto the adjustment layer.
I don’t know what the possibilities are for this in Publisher alone – it doesn’t have pixel-based painting tools – but someone else may know. (If you have Publisher and Photo you can switch to the Photo Persona to do the pixel-based editing and then switch back to the Publisher Persona.)

You’re welcome.
Not everyone looks at the thread title, especially if they come to a post in the thread via a web search.
My intention was to give a bit more information for other people who find this thread when they are looking for the same sort of thing.