This has not been added to the Simulation→Options→Analog form for AMS (for some reason), and so far I've not seen any request to add it (a couple of people have been given the workaround). I suggest you contact customer support and request an enhancement to add this.
The workaround is to use the Miscellaneous tab of Simulation→Options→Analog and then in the "Additional arguments" field, enter something like:
noiseon_type=[thermal]
You can use noiseoff_type too, and the allowed values inside the square brackets are given in "spectre -h options" (or can be seen on the Spectre UI). Note that you would need to select the appropriate instances via the noiseon/off options on the Main tab, as you do with spectre.