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We propose to observe the new bright X-ray transient Swift J1658.2-4242 as soon as feasible during one of the coming revolutions for a total exposure time of ~170 ks in order to probe the nature of the source (the observation can also be split in two revolutions).We note that although the Galactic Center region will be observed during the next revolutions Swift J1658.2-4242 will be always far off-axis and visible only in a very few pointings. This is detrimental for the spectral signal to noise ratio and the instrument calibrations (beside being always outside the JEMX field of view).The observation could be performed together with an already accepted NuSTAR DDT ToO (not requested by our group but the NuSTAR PI can be contacted).Swift J1658.2-4242 is a new transient source in the Galactic plane which has been recently discovered by the Swift Burst Alert Telescope (GCN #22416 GCN #22417). A follow up Swift/XRT observation (ATel #11307) showed that its 0.3-10 keV spectrum can be described by an absorbed power-law with photon index 1.2+/-0.3 and column density of 1.5e23 cm^-2. The source flux is about 7.2 (+/-0.6)E-10 erg/cm^2/s in the 0.3-10 keV.Swift J1658.2-4242 has been observed also by INTEGRAL (ATEL #11306) during an observation of the Galactic center region.The source was always observed strongly off-axis thus outside of the JEM-X field of view. The IBIS/ISGRI fluxes were about 7e-10 erg/cm^2/s (20-40 keV) and 4e-10 erg/cm^2/s (40-80 keV).Kennea (ATel #11311) reported the detection of possible periodicity in Swift/XRT data (exposure: 1629s) of P = 8.7274 +- 0.0078 s indicating the Swift J1658.2-4242 is possibly an accreting pulsar.We propose to monitor the soft and hard X-ray emission of the source throughout the evolution and fall of the outburst to better constrain its nature from its X-ray properties and optimally measure its broad-band X-ray spectrum looking in particular for cyclotron lines. If detec [truncated! Please see actual data for full text]