how can i put a mask on a field?

Depending on your needs you can use the Masked Input widget:

http://widgets.jotform.com/widget/masked_input 

You can test it here:

http://form.jotformpro.com/form/50271133485955? 

An alternative might be using a condition to set a mask to a field if a condition is met:

Let us know if we can assist you further.