It’s not super convenient, but we guarantee that a bias vector of each LSTM layer is structured like this:
[b_ig | b_fg | b_gg | b_og]

You can find that in the Variables section of the LSTM docs 352.
So, to set the forget gate bias, you’d need to filter out the bias parameters, and set all indices from 1/4 to 1/2 of the length to the desired value.