What is the exact effect you're trying to achieve? For example, if you just
wanted to display the selection of each dropdown as text, you could use the
contact function. Suppose your dropdowns are called field1 and field2,
respectively, and the textbox is field3. Use the "Fx" button (formula) in the
properties of field3 to set its value, insert the "concat" function then
specify field1 and field2 as its parameters (you can remove the third
parameter). Or you could put some text (space, dash…) between the two fields
using the same very concat().