In v SP05 you build the filter clause in the URI. I couldn't find the syntax in any documentation or in swagger, so I had to open a case.
You append the following to the url and encode it:
?filter=<property>==<value>
Example:
?filter=runstate == running
Encoded:
$PAMBaseURL = "https://capa.company.com/api/rest"
$URI = "$PAMBaseURL/v2/processinstances?filter=RunState%20%3D%3D%20Blocked"