Now we're going to use a quick edit mask and the paint brush to define a layer mask.
Hit the Q key to enter Quick mask mode and using a normal paint brush with soft edges, paint the area roughly you would like to frost.

Exit quick mask by hitting Q again, then inverse the selection (quick key Shift-Ctrl-I or menu Select>Inverse). Add a layer mask using this selected area.
(or Layer>Add Layer Mask)
Your text should resemble this.

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