Add Power-Mail-in-a-box DNS documentation

dnsapi2.md

@@ -92,6 +92,7 @@
 - [168. Use HE DNS DDNS API](#dns_he_ddns)
 - [169. Using the IONOS Cloud DNS API](#dns_ionos_cloud)
 - [170. Use omg.lol API](#dns_omglol)
+- [171. Use Power-MailinaBox](#dns_pmiab)
 - [Use custom API](#dns_myapi)
 - [Use lexicon DNS API](#dns_lexicon)
 
@@ -2340,6 +2341,27 @@ Please report any bugs or issues [here](https://github.com/acmesh-official/acme.
 
 <a name="dns_myapi"/>
 
+## 171. Use Power-Mailinabox
+
+Use the Power-Mail-In-a-Box (PMIAB) Custom DNS REST API interface to PMIAB DNS.
+You only need to set your PMIAB login credentials and the fully qualified domain name of the PMIAB Server.
+Suggest single quote over double quote to ensure characters are not interpreted by the shell - important for passwords.
+
+```sh
+export PMIAB_Username='your_PMIAB_admin_username'
+export PMIAB_Password='your_PMIAB_admin_password'
+export PMIAB_Server='FQDN_of_your_PMIAB_Server'
+```
+
+To issue a cert:
+```sh
+./acme.sh --issue --dns dns_pmiab -d example.com -d *.example.com
+```
+
+The `PMIAB_Username`, `PMIAB_Password` and `PMIAB_Server` will be saved in `~/.acme.sh/account.conf` and will be reused when needed.
+
+Report any bugs or issues [here](https://github.com/acmesh-official/acme.sh/issues/2550)
+
 ## Use custom API
 
 If your API is not supported yet, you can write your own DNS API.