There should be a "SaveCurrent" in the Preset dropdown. Can't see it in your Screenshot. Do you use the latest version of MM?
This saves a .xml to ...Documents/meshmixer/printers/supportPresets (assuming a default install)
You can edit or add such a .xml manually too. MM needs to be rebooted to show changes in that directory.
Here's the content of such a preset .xml (values not meaningful - just to show if integer or float):
<?xml version="1.0"?>
<SupportStructurePresets>
<Preset>
<Name>SomePresetName</Name>
<PolicyType>0</PolicyType>
<OverhangAngle>0.1</OverhangAngle>
<ContactTolerance>0.1</ContactTolerance>
<VerticalOffset>0.1</VerticalOffset>
<LayerHeight>0.1</LayerHeight>
<SupportMaxAngle>10</SupportMaxAngle>
<SupportDensity>0.01</SupportDensity>
<PostDiameter>0.1</PostDiameter>
<TipDiameter>0.1</TipDiameter>
<BaseDiameter>0.1</BaseDiameter>
<AllowTopConnections>0</AllowTopConnections>
<OptimizationRounds>1</OptimizationRounds>
<SolidMinOffset>0.1</SolidMinOffset>
<TipHeight>0.1</TipHeight>
<BaseHeight>0.1</BaseHeight>
<StrutDensity>0.01</StrutDensity>
<PostSides>1</PostSides>
</Preset>
</SupportStructurePresets>