Absolutely free demo version is available on the downloads page. But demo version has save and export feature limitations.
Note that both Standard and Professional editions of Metashape 1.5 and 1.6 versions allows to run the fully functional trial from the Activate Product dialog without a need of requesting trial key.
Trial license key for Professional edition of Metashape can be requested via online web-form:
The same key is valid for older versions Agisoft PhotoScan Professional.