Nomenclature of unusual nitrogen functional group
According to Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the preferred IUPAC name of the compound $\ce{H2N-CH=NH}$ is methanimidamide. The name formimidamide may be used in general nomenclature. The name formamidine is no longer recommended.
The letter locants ‘N’ and ‘N'’ are used to differentiate among the different nitrogen atoms. Thus, the compound $\ce{R-NH-CH=N-OH}$ would be named as N'-hydroxy-N-…methanimidamide.