Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prosthĕsis, is, f., = πρόσθεσις, and prŏthĕsis, is, f., = πρόθεσις, a grammatical figure consisting in the prefixing of a letter or syllable to a word, prosthesis; as gnatus for natus, tetuli for tuli, Charis. 4; Diom. 2.