Single Word for a Person starting a process or activity

Solution 1:

Two words that might fit:

1. initiator:

   A person or thing that initiates someone or something.

   (source: Lexico)

2. starter

   A person or thing that starts in a specified way.

   (source: Lexico)

Solution 2:

originator (n.)

someone or something that originates or gives rise to something m-w

A person who creates or initiates something. Lexico

The reply stimulated by the request chain is routed to the originator of the chain, along the same path, by successively peeling addresses off the stack. ACM Transaction on Software Engineering and Methodology

To help determine the authenticity of these messages always go back to the originator of the chain and ask about the story John Williams; The Cost of Deception

The supervisor, in turn, will be responsible for contacting the originator of the comments to amend the ... Brenda Potter; Medical Office Administration

Solution 3:

You are using this in a technical context, and besides the general user, there are two terms of art that may apply:

• actor, if the focus is on the person physically taking action:

  A Technical Actor represents the types of interactions with the system. An actor may assume one or more roles, and can be equated to the external people or systems that interact with the system under study. Note that Actors are not individuals nor are they necessarily equivalent to job titles; instead, they describe the behavior in the enterprise and the responsibilities of the associated "user". —Unified Architecture Method

• principal, if the focus is on the authority for the action (which in some cases is not the same as the actor):

  Security principals are any entity that can be authenticated by the operating system, such as a user account, a computer account, or a thread or process that runs in the security context of a user or computer account —Microsoft

See also this discussion on SO.

Solution 4:

Perhaps instigator:

A person who brings about or initiates something. (Lexico)

Solution 5:

There is also

pioneer
1.1 A person who is among the first to research and develop a new area of knowledge or activity.
Lexico