This work describes the in-situ CTR plugin for the particle-in-cell code PIConGPU. The C++ -plugin calculates coherent transition radiation (CTR) from millions to billions of macro-particles in a PIConGPU plasma simulation. In order to avoid excessive disk output of many GB to TB of data, as well as and extensive post-processing runtimes on only few CPUs, the plugin was parallelized on GPUs and implemented in-situ as part of PIConGPU. The physics of this plugins was successfully implemented, tested and verified to agree with an initial python implementation, which again was verified using analytical CTR theory, with an average error of less than 1%. Additionally the plugin was benchmarked, resulting in typical time to solutions for complete transition radiation spectra on the scale of several minutes. The CTR plugin was then used in several Laser-wakefield accelerator (LWFA) simulations, with self-injected and down-ramp injected electrons respectively. Mimicking the hypothetical placement of successive transition radiaton foils along the LWFA interaction length, the CTR plugin was used multiple times for observing how the electron bunch evolution leads to the characteristic features of transition radiation spectra. This new tool is a synthetic diagnostic, which enables direct comparison of experimentally measured transition radiation data to simulations. In future experiments this can provide insight into LWFA longitudinal electron pulse profiles on the fs-scale, required for future compact LWFA-driven free-electron laser applications.