I build store to Streamline theme. I noticed only problem with the theme's contact form which doesn't include subject field.
Name, email and message -fields are as they should. I tried to use contact form -App, but it doesn't worked with template suffix: contact.page.
Could someone help how to add subject field to the contact form?
The contact form doesn't support a custom subject message so not seeing that option in your theme does make sense.
You did mention app in your post though - are you not using the default Shopify form but instead using a third party one?