Answer
There's no fully generic mechanism for this across every page, but some Assistants specifically support passing initial field values through their own URL parameters — for example, an assistant's URL can include an $action=InitConfig{...} query segment naming specific fields (like a Customer ID) to pre-populate. The exact list of supported parameters for a given assistant is defined in that assistant's own client-model fragment file, so check the specific assistant's fragment (or its technical documentation) to see what it accepts before assuming a field can be pre-filled this way.