Hey Jorge,

So the "native" way is to use the calculation field in the advanced tab of the field you want. to be the value of the drop-down. Like below:

If you click the > it will give you a list of the inputs of the form you have. Select the dropdown's variable (whatever you named it) and it will populate as the value of the dropdown. The nice part of doing this is it also updates as you change the dorp-down. 

There are ways of doing this in JavaScript, but this is the simplest (if it works) and requires no coding at all.

Hope that helps.