I'm going to assume you're doing an anova. But I was able to fake it by putting a 0 line width reference line at the spot where I wanted it, then you can do whatever you want to the font.
Names default to here( 1 );
dt = open("$SAMPLE_DATA\Big Class.jmp");
dt << Oneway(
Y( :height ),
X( :age ),
SendToReport(
Dispatch(
{},
"2",
ScaleBox,
{Add Ref Line(
21,
"Solid",
"Black",
"14",
0,
1,
Label Settings( {Set Font Style( "Bold" )} )
)}
)
)
);
I got the number 21 from inside the value labels of the axis.