According to Custom query parameters, parameters with the "ext—" prefix can be passed to the "/authorize" endpoint and then used in the universal login page templates.

The usage of `ext-` parameters comes with a few limits, though. This article details those limitations

• /authorize endpoint
• Customize Universal Login Page Templates
• "ext—" prefix
• Limitations

The usage of `ext-` parameters has the following limits:

1. The `ext-` parameter names must be unique
2. There could be max 10 `ext-` parameters in one `/authorize` request
3. The `ext-` parameter name must match this regex `/^ext-[\w-]{1,28}$/` (start with `ext-`, contains [a-zA-z0-9_-] only, and max 28 characters)
4. The `ext-` parameter value must match this regex `/^[-\w.*~@+ /:]{1,255}$/` (contains [a-zA-Z0-9-.*~@+ /:] only, and max 255 characters)

If the limits are exceeded, the following errors could be returned:

// limit #1 exceeded
error parsing ext params: must have less than or equal to 10 keys with "ext-" prefix
// limit #4 exceeded
error parsing ext params: "ext-XXXX" with value XXXXX fails to match the required pattern: /^[-\\w.*~@+ /:]{1,255}$/'`
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