SnapGene cannot currently search for, or retrieve, NCBI Assembly records or Assembly Accession numbers.
However, if an assembly comprises a complete genome then it may also have a corresponding GenBank Accession that you can retrieve. Locate the Assembly at https://www.ncbi.nlm.nih.gov/ to find the GenBank Accession number. For example, NCBI Assembly ASM1316697v1 (Assembly Accession number GCA_013166975.1) has a corresponding GenBank accession CP053601.1.
You can search and retrieve the corresponding GenBank Accession CP053601.1 from SnapGene via menu File → Import → NCBI Sequences.
The genome will import and open in SnapGene.