In the Map Defaults page of the MapMixer Configuration there is a button on the left bottom labelled "ONS Default Links", which opens up a separate menu where you can override Onslaught default link setups.
When creating your own Map List, you can add an Onslaught map along with a specific link setup, however, if you are using the MapMixer Auto Create Map List option with the Onslaught game type there may be no way to specify a new link setup for an Onslaught map. You could set up a custom map list as a game source in your Games List, however this may not always be the best solution.
In any case, you may just simply want to override the default link setup for an Onslaught map, so that whenever it is chosen (without link preference) it always uses a given link setup.
Simply click on a map and choose a default Link Setup in the drop-down list below. Now whenever this map comes into play in the Onslaught game type (where no Link Setup specified) then the Link Setup you chose here is used instead. Note that if you selected the "Random" link setup here, then every time you played an Onslaught map where no link setup has been specified then a random will be chosen.