https://wiki.opendaylight.org/view/BGP_LS_PCEP:Programmer_Guide#Creating_LSP_2
specifies the new way of add-lsp using XML input, the change from Helium is that
top-level <input> needs to have namespace specified explicitly (required in Lithium).

This format looks like it should also work on Helium, but it leads to an error on sub-element "lsp":
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<errors xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf">
<error>
<error-type>application</error-type>
<error-tag>unknown-element</error-tag>
<error-message>Schema node "lsp" was not found in module.</error-message>
</error>
</errors>

Analogous error is present also in master branch, but Helium-SR3 has deadline sooner.