This paper considers a diffusion-based molecular communication link where both the transmitter and receiver are molecular circuits. We define the input (resp. output) signal as the number of signalling molecules (output molecules) at the transmitter (receiver) over time. For a given receiver circuit, an open research problem is to find the transmitter circuit that maximises the mutual information between the input and output signals. This general problem is hard to solve. We simplify the problem by: (1) Specialising to specific receiver molecular circuits; (2) Parametrising the transmitter by a pre-defined set of chemical reactions which gives the input signal some specific spectral characteristics. We then use numerical optimisation to determine the transmitter parameters that maximise the mutual information. We show that our parameterised transmitter circuit is able to give a mutual information which is close to upper bound obtained in our earlier work.