I still want the sensible suggestion of making forms available to submit via the Global issue create button / form.

There are currently two options that don't require addons though:

1) I'm not sure how far it's been rolled out but we already get a warning that a Form is attached to a request type which provides the link to the portal form:

This request type includes a form that can’t be seen in this view

Create an issue using all fields on this request type and its form in the portal.
Open Portal

2) In the form settings there is also this option:

Create new issue: Use a link to this form to create new issues in Jira Service Management.

Without going to the portal view agents can go to https://yoursite.atlassian.net/jira/forms/create to submit issues using any form that has the 'Create new issue' option ticked.
 
This option doesn't include any request type fields that don't appear on the form so if you have validation on forms that don't appear on the form you won't be able to submit it.
 
This does still however include any hidden preset fields that your Request Type has configured which is handy.

A possible workaround, using Scriptrunner behaviours:

Make the Summary field read-only, and add a field help text that links to the ProForma form:

def formField = getFieldById(getFieldChanged())formField.setHelpText("Please use <a href=\"https://link-to-proforma-form\">this form</a> create your issue.")

It's not the prettiest of solutions, and this suggestion would be nice to have implemented, but it works

This is actually a big problem for my company. We use ProForma forms because having too many custom fields will slow down our Jira instance. But we still need to use some Jira fields like Priority or Attachment because either its not an option on the forms or because of the time it takes for the form to update linked fields. On our customer portal this isn't a problem since the form and the Jira fields can both be used. But several of our teams have internal helpdesk projects and there is no way internally for our agents to create issues in these projects with forms and Jira fields unless they go out to the portal. And honestly I'd rather they do that since the portal allows me to list the different request types and explain what they are but in the menu bar is a button labeled "create". This button allows the agents to create issues but it only shows the fields listed in the issue type's create screen. There are a whole lot of other questions that we'd like to ask but because the form isn't included they don't get answered. No matter how much training we do, it is very hard to stop people from using the create button. It's convenient and very visible.

Since ProForma forms seem like a very necessary and handy way to set up issue creation, why aren't they included on the internal create issue view? Why aren't only "customers" allowed the convenience of ProForma forms and Jira fields? Sure the agent can add it to the issue after the fact, but they don't always know what form is the right form and there is nothing in the lists of forms other than the name to help them figure it out.