How do I format a SharePoint list column input to complete a URL?

RogueWolf1985 Yes, this is possible. You can use below JSON code on your ProjectName column:

{
    "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
    "elmType": "a",
    "txtContent": "@currentField",
    "attributes": {
        "target": "_blank",
        "href": "='https://company.sharepoint.com/sites/ParentSite/' + @currentField"
    }
}

Also, if "https://company.sharepoint.com/sites/ParentSite" is the same site where you are adding this JSON code then you can also use "@currentWeb" instead of hard coding the site URL in JSON like below:

{
    "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
    "elmType": "a",
    "txtContent": "@currentField",
    "attributes": {
         "target": "_blank",
         "href": "=@currentWeb + '/' + @currentField"
     }
}

Use column formatting to customize SharePoint 

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