the graphical user interface will let you define a model like this first define WT as a covariate then map it if the name in your data is not WT otherwise it will be automatically mapped. By default the covariate is continuous and you can just type in any value for centering e.g. 70
the code will look like
…
fcovariate(WT)
stparm(Cl = tvCl * (WT/70)^dCldWT * exp(nCl))
and then you can “freeze” the dCldWT to 0.75
or then go to textual mode and edit it to and remove dCldWT parameter