You can use technical fields names in Input Sets instead of the field labels.

An alternative is to define the Input Set in your GuiXT script with

Image (toolbar) xxx.gif   input="..."

Here you  can use either your own icon "xxx.gif", size 18x18 pixel, or you can refer to the "letter" icons that GuiXT displays for Input Sets. The notation "x.res" displays the blue letter icons (normal Input  Sets) and "x.resc" the yellow icons.

Image (toolbar) "d.resc" input="ok:/NSE37"