According to the authors, "The primers used in this study were cIslet1 (forward, ATCGCCGGGGATGAGCTGGCGGCT; reverse, TCAAACCTACTTTGGGGTCTTAAA; amplifying 426 nt at positions 211636 nt; Tanabe et al., 1995)." The sequence below was obtained from NCBI (NM_205414) using this information.
Images and probe sequence provided by Claire Anderson. Related paper: Anderson C, Hill B, Lu HC, Moverley A, Yang Y, Oliveira NMM, Baldock RA, Stern CD. A 3D molecular atlas of the chick embryonic heart. Dev Biol. 2019 Dec 1;456(1):40-46. doi: 10.1016/j.ydbio.2019.07.003. Epub 2019 Jul 5.