If I Add build step → Execute system Groovy script → Groovy command:

import hudson.model.*

// get parameters
def parameters = build?.actions.find{ it instanceof ParametersAction }?.parameters
parameters.each {
   println "parameter ${it.name}:"
   println it.dump()
   println "-" * 80
}

// ... or if you want the parameter by name ...
def hardcoded_param = "FOOBAR"
def resolver = build.buildVariableResolver
def hardcoded_param_value = resolver.resolve(hardcoded_param)

println "param ${hardcoded_param} value : ${hardcoded_param_value}" 

the output is:

param FOOBAR value : null

which is comprehensible, since where is the value of ${FOOBAR} supposed to come from?

If I select ☑This build is parameterized → Add parameter → String Parameter:

• Name: FOOBAR
• Default Value: FOOBAR_value

the output is:

parameter FOOBAR:
<hudson.model.StringParameterValue@a6648c13 value=FOOBAR_value name=FOOBAR description=>
--------------------------------------------------------------------------------
param FOOBAR value : FOOBAR_value