Hi @OCappelli,
It seems like your form provider is passing on the field label to HubSpot (which is then not recognized), not the field name. This is unfortunately something you’d have to resolve with the forms provider.
You can test whether this is the issue by changing the field label to “firstname”. If the submission goes through correctly, that’d be the issue.
Alternatively, you could consider setting up and embedding a HubSpot form on your page to avoid this.
Best regards