You’ll need to set the visualization to Custom and then change the band selector to the script option.

If you want to see the values as panchromatic, you can just use:

return n(B08 - B05) / (B08 + B05)];

Or you can use the Colormap or Colorblend functions to use a color ramp:
var val = (B08 - B05) / (B08 + B05);
return colorBlend(val, ,-0.2,0,0.2,0.4,0.6,0.8], , 0.447,0.243,0.039],]1,0.1,0],]1,1,0],]0.1925,1,0.141],]0.043,0.49,0.043],]0.047,0.082,0.424]]);

See example.

Hi, Thx. for your reply and example. The Spatial resolution of the 2 Bands are different, B08 (NIR) is 10 m. and B05 (Near-edge) is 29m. - How to account for the difference ?