Below are two useful links regarding applicable regulations, guides and information needed for POA processes:

Civil aviation production organisations, which intend to produce products, parts and equipment accompanied by the certificate of release to service EASA Form 1. The production of these products, parts and equipment is to be appropriate under approval Part 21 Subpart G, as set out in regulatory point 21.A.133 of Part 21 Subpart G of Annex I to Regulation (EU) 748/2012. Only for direct delivery to final customer (e.g. air operator, maintenance workshop, etc.).

Approval of Production Organisation as described in Part 21 Subpart G of Annex I to Regulation (EU) 748/2012. Applicable for those production activities falling within the scope of work of EASA, as set out in Article 2 of Regulation (EU) 2018/1139.

As can be seen on the relevant EASA website (ETSO FAQ), you must be, or have previously applied for, the ADOA (or DOA, if it is an ETSO article which is an APU) and the POA.
 
The DOA/ADOA and ETSO are held by EASA. The POA is handled by AESA as the Spanish Aviation Authority. Therefore, all 3 approvals have to be requested at the same time (approximately), from the respective Authority.
 
A.133 (Eligibility) is essential to remember in relation to the POA:

An Airworthiness Certificate is the document that accredits the aircraft to which it refers, as being in a position to fly by meeting certain safety requirements. 
AESA would be the authority responsible for issuing Airworthiness Certificates for aircraft that are or will be registered in Spain. 

The Station License is mandatory for any aircraft with Spanish registration that has emitting aeronautical equipment.

GPS equipment should not appear on the station license.

A Flight Authorisation (also called Flight Permit) is a permit that is normally issued when the Airworthiness Certificate is temporarily invalid and the aircraft is required to fly for established reasons. 

Non-EASA aircraft, are those types of aircraft by which they are excluded from EASA Regulation 2018/1139. Among these types are ultralight motorised aircraft called ULM, Historical, Experimental, Amateur Construction Aircraft. Also excluded from the EASA regulations are those aircraft that are used in the use of military operations, patrol boats, police, etc. For more details of which aircraft are excluded, you can consult Regulation 2018/1139, in its Article 2.3(a) and Annex I.

WIFI equipment should not appear on the station license.