How can I add multiple bootstrap hosts in the MarkLogic cluster using MarkLogic API?

Following case worked in Postman:

HTTP Verb:
PUT

Authorization:
Digest Auth admin:admin

Header: 
Content-Type application/json

URI:
{ml-host}:8002/manage/v2/hosts/{new-bootstrap-host}/properties

Body:
{
    "bootstrap-host": true
}  

I will assume you are referring to the Management API. If that's the case, then you can POST against the hosts endpoint to define bootstrap hosts

https://docs.marklogic.com/REST/POST/manage/v2/hosts

cat myHost.json
==> 

{
  "host-name": "hostname1",
  "group": "",
  "bind-port": 8090,
  "foreign-bind-port": 9091,
  "zone": "",
  "bootstrap-host": true
}

curl -X POST --digest -u admin:admin -H "Content-type: application/json" \
-d @myHost.json http://localhost:8002/manage/v2/hosts 

==>  Defines the host, named "hostname1," as the bootstrap host in 
     the cluster. 

Bootstrap host -> dh5a

Target joining host -> dh5b

1. Save dh5b config as dh5b-config.xml

curl -o dh5b-config.xml --user {authen-user:passwd} \
-X GET -H "Content-type:application/xml" \
http://dh5b:8001/admin/v1/server-config

2. Send dh5b config to dh5a as cluster-config.zip

curl --digest --user {authen-user:passwd} -X POST -o cluster-config.zip -d "group=Default" \
--data-urlencode "server-config@./dh5b-config.xml" \
-H "Content-type: application/x-www-form-urlencoded" \
http://dh5a:8001/admin/v1/cluster-config

3. Send cluster-config.zip to dh5b and complete the joining process

curl --anyauth --user {authen-user:passwd} -X POST -H "Content-type: application/zip" \
--data-binary @./cluster-config.zip \
http://dh5b:8001/admin/v1/cluster-config

For production deployment:

1) Use Gradle to deploy additional hosts if Gradle is permitted

2) Write/Execute Shell script to include above API operations

(This approach has a lot more fun: combination of old Shell and modern API)

You can then use automation tool to invoke Shell or Gradle.