You could use Wiql - Query By Wiql API like below to get the code reviews:
POST https://dev.azure.com/{organization}/{project}/_apis/wit/wiql?api-version=5.1
# get all code review that I request
{
"query": "SELECT [System.Id], [System.Links.LinkType], [System.Title], [System.State], [System.Reason], [System.AssignedTo] FROM WorkItemLinks WHERE (Source.[System.TeamProject] = @project and Source.[System.WorkItemType] in group 'Microsoft.CodeReviewRequestCategory' and Source.[System.AssignedTo] = @me and Source.[Microsoft.VSTS.Common.StateCode] <> '1') and ([System.Links.LinkType] = 'System.LinkTypes.Hierarchy-Forward') and (Target.[System.WorkItemType] in group 'Microsoft.CodeReviewResponseCategory') ORDER BY [System.CreatedDate] desc"
}
# get all code review that I you are a reviewer
{
"query": "SELECT [System.Id], [System.Links.LinkType], [System.Title], [System.State], [System.Reason], [System.AssignedTo] FROM WorkItemLinks WHERE ([Source].[System.TeamProject] = @project AND [Source].[System.WorkItemType] IN GROUP 'Microsoft.CodeReviewRequestCategory' AND [Source].[Microsoft.VSTS.Common.StateCode] <> 1) And ([System.Links.LinkType] = 'System.LinkTypes.Hierarchy-Forward') And ([Target].[System.WorkItemType] IN GROUP 'Microsoft.CodeReviewResponseCategory' AND [Target].[System.AssignedTo] = @me) ORDER BY [System.CreatedDate] desc"
}