Any reason not to just make Form an optional param on invoke-boxrestcall and simplify this?

No preferences here, just something I didn't think about doing and would be happy to look at this as an improvement in the near future

This example seems to include a parameter that isn't valid, and seems to omit several parameters that are marked mandatory.

Just updated this, nice catch!

Making two arrays that match exactly to set permissions seems like it could be error prone if there are very many collaborators? Getting off by one might set a lot of incorrect permissions. You could consider for a future improvement taking an powershell object with key value pairs for the user and role such as:

$obj1 = {
    user = "user@example.com"
    role = "reader"
}

Or alternately, parameters Login and Role could be set to only accept a single string instead of an array and then use ValueFromPipelineByPropertyName so that objects could be sent from the pipeline.

I initially had tried the powershell object, but for some reason, i couldn't get it to send the login and role properly, this is something I would definitely like to improve on.

Would this be more modular if it was a separate function? i.e. Have a function for New-BoxFolder and then turn this function into something like New-Collaborator (or whatever the proper pwsh verb might be)?

This is probably the way, this was created initially with a very specific need in mind, but breaking is probably the way to go.

That would also allow us to in the future add collaborators to an existing folder.

Splitting into two functions and then using ValueFromPipelineByPropertyName for the add collaborator piece would keep the function login much simpler as well.

Just curious, Does this work if using the WhatIf flag? $FolderId wouldn't exist at this point, right? Or am I thinking about this wrong?

This does indeed for example, it would look like:
What if: Performing the operation "Create folder 'BoxAPITest1'" on target "Box".