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Young massive clusters (YMCs) are the most massive star clusters forming in nearby galaxies and are thought to be a young analogue to the globular clusters. Understanding the formation process of YMCs leads to looking into very efficient star formation in high-redshift galaxies suggested by recent JWST observations. We investigate possible observational signatures of their formation stage, particularly when the mass of a cluster is increasing via accretion from a natal molecular cloud. To this end, we study the broad-band continuum emission from ionized gas and dust enshrouding YMCs, whose formation is followed by recent radiation hydrodynamics simulations. We perform post-process radiative transfer calculations using simulation snapshots and find characteristic spectral features at radio and far-infrared frequencies. We show that a striking feature is long-lasting, strong free–free emission from a ∼10-pc-scale H  ii region with a large emission measure of ≳107 cm−6 pc, corresponding to the mean electron density of ≳103 cm−3. There is a turnover feature below ∼10 GHz, a signature of the optically thick free–free emission, often found in Galactic ultracompact H  ii regions. These features come from the peculiar YMC formation process, where the cluster's gravity effectively traps photoionized gas for a long duration and enables continuous star formation within the cluster. Such large and dense H  ii regions show distinct distribution on the density–size diagram, apart from the standard sequence of Galactic H  ii regions. This is consistent with the observational trend inferred for extragalactic H  ii regions associated with YMCs. [ABSTRACT FROM AUTHOR]