When using resulting URIs as input for another API method, note that the API call for MapURL uses "Uri" (a capital U) as the name of the input query whereas all other input fields that accept URIs are labelled "uri" (not capitalized).

Several API methods use strings rather than URIs as input.  They are: 

Chemical Structure Conversion calls. In these calls the query field is named either, inchi, ichi_key, or smiles

Chemical Structure Search. In these calls the query field is named serachOptions.Molecule

Free Text to Concept. The query field is labeled "q"

Free Text to Concept for Semantic Tag. 2 query fields are required, "q" and "uuid"

Concept Description. The query field is "uuid"