The solder mask is automatically generated for Gerber production, but you can add it is a layer for editing within DesignSpark PCB. With the solder mask layer enabled you will be able add shapes to the layer and exclude the mask from specific areas such as under thermal tabs.
Launch the Design technology Shift + T and click 'add' and fill out the details as shown.
Mover layers in the stack for the correct order they physically appear in. (Note 'documentation' layer is wrong in attached image it should be at the top).
Using the interaction bar you will see your new layer as shown.
Repeating the above selecting the appropriate details you can created a bottom solder mask.
NOTE: At the start of a PCB design when using the wizard you can select to have solder mask and solder paste mask layers enabled on required layers to have them automatically available for editing.
How do I create a fiducial mark?
How can I edit the mask aperture settings?