The Scenario
You are on the marketing team launching a new product line. Before the first subscriber import runs, you need six custom fields created in MailerLite: account tier, signup source, last purchase date, city, company size, and industry.
The field definitions are in an Excel workbook. Column A has the field name, column B has the data type (text, number, date).
The bad version:
- You log into MailerLite and navigate to Subscribers → Fields
- You click "Add new field," enter the field name, pick the type, save
- You repeat this six times
- You realize you spelled 'signup_source' as 'signup-source' — the import template expects the exact key
- You delete the field and recreate it
- Your first import has the wrong field key and none of the data lands in the right place.
The fast version creates the fields from the workbook and writes the exact field keys back so the import template is built from what MailerLite actually assigned.
The Easy Way: One Prompt in SheetXAI
SheetXAI is an AI agent inside your Excel workbook that creates MailerLite custom fields in bulk and writes the created field keys and IDs back.
Open the SheetXAI sidebar and type:
Create a new MailerLite custom field for each row in my workbook using the field name in column A and data type in column B — write the field key and ID into columns C and D.
SheetXAI reads the six rows, calls the MailerLite custom fields API for each, and writes the created key and ID into columns C and D.
What You Get
Six MailerLite custom fields, with keys and IDs written back to the workbook:
- Field names taken directly from column A — no retyping
- Data types set per row from column B
- Field key written into column C — the exact string to use in imports
- Field ID written into column D — for any API work that references fields by ID
The field key is the part that trips people up. MailerLite normalizes names into snake_case keys, and if your import template uses different casing, the custom field data is silently dropped. With the key in the workbook, you copy it directly into the template.
What If the Data Is Not Quite Ready
Custom field definitions evolve during setup. SheetXAI handles variations in the same prompt.
When some field names have spaces that need to become underscores
The workbook was filled out by a stakeholder who used spaces ("Last Purchase Date" instead of "last_purchase_date").
For each row, convert the field name in column A to snake_case before creating the MailerLite custom field. Write the snake_case name, field key, and field ID into columns C, D, and E.
When some fields already exist and you only want the missing ones
You ran this last month for three fields. The workbook now has six and you only need the three new ones.
Check each field name in column A against existing MailerLite custom fields. For fields that already exist, write ALREADY_EXISTS and the existing key and ID into columns C and D. For fields that do not exist, create them and write CREATED, the field key, and ID into columns C, D, E.
When you want to verify the fields are live immediately after creation
The team wants confirmation that the fields are active before the import template is distributed.
Create a MailerLite custom field for each row in column A using the data type in column B. Write the field key into column C and field ID into column D. Write CREATED into column E for each row to confirm the field is active.
When the field list comes from two tabs and you want a consolidated record
The Product team defined three fields, the Marketing team defined three more, each in a different tab.
Combine the field definitions from the Product Fields tab (columns A and B) and the Marketing Fields tab (columns A and B) into one list. Create a MailerLite custom field for each. Write the field name, field key, and ID into this Summary tab — one row per field.
The pattern: build the field record in the workbook before the import runs. The keys in column C are the source of truth for everything downstream.
Try It
Get the 7-day free trial of SheetXAI and open any Excel workbook with custom field definitions, then ask it to create the MailerLite custom fields and write the keys back. The MailerLite integration is included in every SheetXAI plan. For related workflows, see how to bulk import subscribers into MailerLite from Excel or the MailerLite in Excel overview.