I recommend that you create a single line of text column first then convert this column into hyperlink formatting.
Please follow below steps.
1.Create a single line of text column in the list. Then set default value for the column.
2.Use JSON codes to format this column.
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "a",
"txtContent": "@currentField",
"attributes": {
"target": "_blank",
"href": "https://learn.microsoft.com/en-us/answers/questions/852560/sharepoint-default-link-value-in-list.html"
}
}
3.Result.
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